Well, just remember that they hide illness, so it is better to be overly cautious than under cautious...if symptoms show up, it is often because they have already been sick for a period of time. See how she continues to act, but that's odd for it to happen out of the blue when it has never happened in 4 months. That seems really strange....and does she normally eat during that time? I mean, how often does she eat and what does she eat?
They are both sexually mature, so she could possibly be having an issue with an egg....look up egg-binding so that you are aware of the symptoms.
It could be so many things: bacteria, a virus, something in the air, something internal, a blockage, an egg, a nutritional issue, yest/fungus, parasite....I mean, the list of possibilities is huge without a vet (if there is something wrong).
If she is sick, DO NOT assume that your boy bird is healthy, as birds can be carriers of viruses etc without showing any symptoms. SO, if you find out what is wrong with her (if there is an issue) don't just assume that he is fine because he is acting healthy.