I highly-suggest that you get her in to a Certified Avian Vet or an Avian Specialist Vet immediately, as soon as you can, or even into an Exotics Vet as long as they are willing to take a culture and send it out to a lab, and to do microscopy/gram-staining of it in the office on the spot...You need to demand that...
This is the problem with Exotics Vets, they aren't comfortable treating small birds, so they simply prescribe a broad-spectrum antibiotic and that's it, no diagnostic tests at all...And the problem is that the majority of the time they either give the wrong antibiotic to treat the bacteria causing the infection (usually they prescribe Baytril (Enrofloxacin), which doesn't treat a lot of common Avian bacterial infections, specifically Psittacosis Chlamydia), OR the poor bird doesn't even have a bacterial infection at all, but rather a Fungal/Yeast infection, as birds do commonly get Fungal infections of the respiratory tract/sinuses...And if that's the case, then the antibiotic they go ahead a prescribe without knowing what is actually wrong with the bird will make the Fungal infection the bird actually has much, much worse, because it kills-off all of the healthy bacteria that normally lives inside of the bird and that keeps Fungi/Yeast from growing in warm, dark parts of their body, specifically in their GI Tract, their mouths/beaks, and in their Sinuses...
So without a simple swab/culture/gram-stain/microscopy, which this vet could have easily and cheaply done, your bird may very well be taking the wrong medication for what the actual problem is...And just as an FYI because I'm not sure you're aware of this or not, due to the comment you made about how "This just started a day or two ago, so it's not like he's been sick for a long time"...It's important that you know that ALL BIRDS/PARROTS posses a natural, innate survival-instinct that causes them to hide ANY AND ALL outward signs/symptoms of illness and/or pain for as long as they possibly can, usually at least a couple of months, sometimes for months to over a year.
So by the time we as people first notice that something is wrong with our birds and that they may be sick or injured/in-pain, they've already been sick for quite some time,
and by the time we first see that they are sick, they are usually VERY SICK, often being too late to even help them. They do this as a protective-mechanism against predators. Since birds are "Flock Animals" and they travel and live in large Flocks of birds, when even a single bird in a flock appears to be sick, injured, or weak, predators look for this and pick-up on it immediately, and this puts the entire flock at risk for being killed...So not only can appearing outwardly sick or injured cause a bird and/or their entire flock to be targeted by predators and then ultimately killed and eaten, but it also causes the rest of their flock to chase them off and abandon them, because they don't want any predators to target them due to the sick bird...So as a result, ALL BIRDS possess an amazing ability to suppress and hide any and all signs and symptoms of illness for an amazing amount of time...
***So the point is that even though you first noticed that your bird's nostril was swollen and clogged a few days ago, the fact of the matter is that she has probably been suffering from a Sinus Infection/Respiratory Infection for at least a month or so...Luckily she hasn't started coughing, wheezing, or having breathing problems yet, because when they get to the point where they are outwardly displaying those types of symptoms they are extremely ill...You did say she had "just started sneezing a bit" a day or so ago, so that's not good...
I'm sorry that the Vet you took her to didn't do their job properly, that's not your fault, but you do need to get her to another Vet or even take her back to that same Vet and REQUEST/DEMAND that they take a swab/culture from the blocked-nostril, and then not only look at it under their microscope in the office right then and there and do a Gram-Stain of the swab so that they can confirm or rule-out a Bacterial Infection, a Fungal/Yeast infection, and/or a Protozoan infection, and if they do see abnormal/negative Bacteria present so that they can identify exactly what strain of Bacteria it is, and prescribe the correct antibiotic that it will be sensitive to...And also have the send the swab out to the lab they use so that they plate it and run a "Culture and Sensitivity"...This is where they put the swab on an agar-plate, put it in an incubator, and let it grow-out, usually for 48-72 hours. This is the only way to diagnose a lot of Fungal infections, as well as some Bacterial infections, because some have to actually have time to grow-out...Then they'll call you when they get the results of the Culture and Sensitivity in a couple of days, and if they have to change the medication they give you because it's the wrong one they can...Either way, just prescribing a broad-spectrum antibiotic and "hoping" it's the correct medication is terrible practice, and is responsible for killing many, many birds every day...
I'd also run to Petco and buy a bottle of Qwiko Avian Probiotics. It costs $9.99 and is quite a large bottle of Probiotics...It comes with a pre-measured scoop inside, and all you do is sprinkle the pre-measured amount of Probiotic powder on top of his seed-mix or pellets once every single day. If you give it to your bird once every day, a bottle will last you a few months, it's a very small amount they get each day...Birds have extremely sensitive GI Tracts, and anytime they are put onto any type of Antibiotics they need to also be put on a daily Probiotic, and they should continue to be given the daily Probiotic for a good 2 weeks AFTER they finish the round of Antibiotics...The Probiotic does two things: #1) It replenishes the normal, "healthy" Bacteria that normally lives throughout their GI Tracts and that is quickly killed-off by the Antibiotics, causing the bird to develop a Fungal/Yeast infection secondary to the antibiotics (which VERY OFTEN results in the bird developing a Fungal/Yeast infection throughout their GI Tracts, as well as in their mouths/beaks/sinuses (called "Oral Thrush"), and then #2) The Probiotics are extremely important if your Vet doesn't run any diagnostic testing at all to determine if your bird even has a Bacterial infection but still just gives them an antibiotic anyway; the Probiotic will not treat your bird if she actually does have a Fungal/Yeast infection instead of a Bacterial infection, if that's the case she needs to be put on a prescription Anti-Fungal medication, however, the Probiotic does definitely help to keep any active Fungal/Yeast infections from spreading/getting worse...And since this Vet just guessed as to what was wrong with your bird and prescribed a no-doubt strong, broad-spectrum antibiotic without knowing what is actually wrong with your bird, the Probiotics are a must...They will also keep your bird's stomach/GI Tract settled, as Antibiotics do often upset their stomachs and cause diarrhea/runny stools...
I have been giving all of my birds a daily Probiotic for years; I use the Qwiko Avian Probiotic and simply sprinkle it right on top of the small portion of healthy seed-mix that they each get every day (it mixes better with seeds than with pellets, but if your bird only eats pellets then that's fine, just sprinkle it over her pellets). None of my birds have ever developed any type of Bacterial or Fungal infections at all, and they all eat very well and their droppings are all normal...Probiotics can't hurt them, they can only help, and I actually take one myself every day as well...But when your bird is on an Antibiotic they are a must; and again, they should get the Probiotic once a day throughout the time they are taking the antibiotic, as well as for a week or two after they finish the antibiotic...
Seriously, you definitely need to get a swab/culture done on your bird to make sure that it is in-fact a Bacterial infection and that the antibiotic they put her on is one that will treat the specific Bacterial infection she has...Vets do this all the time with birds and also with reptiles, like they're not worth actually diagnosing or something, it's okay to just "guess" what's wrong with them...And like I said, the big issue with this is that your bird has most-likely been sick for quite some time already, so if she's not taking the correct medication she's only getting rapidly worse...
