Could be quite a few things. Allergies (to fleas or food, they can come on suddenly, might want to consider switching to a food with no corn or wheat as those tend to be the main allergens, also could be an alleric reaction to cleaning products), ringworm, thyroid problem, mange, as well as a whole list of things that I can't think of at the moment, but that should be a pretty good start. First thing to do would be to take the cat to the vet and have her checked for ringworm, mange and do a blood test to check her thyroid. If those turn out to be ok, I would get some flea drops from the vet (even if you see no fleas, it only takes one flea to start a reaction in an allergic cat) and switch her food, depending on what you're feeding her now. I can give you quite a few suggestions for a good, allergen-free food that won't be too expensive. I wouldn't try her on the allergy food from the vet at first as it's very expensive and if it's a corn allergy, you can find a good quality food with no corn for quite a bit less than what you'd pay at the vet. You can always go for allergy tests, but those are fairly expensive, and the elimination method usually works pretty well.
