Good points that it may just be a dust issue, or even with the changing weather, things in the air like different pollens, etc.
I reserve an antibiotic for EMERGENCY use only as antibiotic resistance isn't something to take lightly. When an antibiotic is given it will be in your litter and the ground in your runs, etc. from droppings. So it doesn't just effect one bird; it effects the environment that they are all in by encouraging antibiotic resistant bacteria in your animal environment.
Then there is the issue that Kab discussed - getting gut flora (where the immune system gains much of it's strength) out of balance which puts the animal at greater risk of not being able to resist other issues when it becomes over populated with yeasts that weaken the animal even more. It's like a vicious circle.
I highly recommend Avi-Culture II avian probiotics that can be added to feed either occasionally or to rebuild after an antibiotic. It is the ONLY avian probiotic on the market that is NOT GROWN ON A GMO SUBSTRATE. They often claim 'GMO FREE' but they are not taking into account the SUBSTRATE on which the bacteria is grown when they state that. When asked directly, there is no manufacturer of probiotics that would state that the substrate was not GMO. Except for AVI culture. When it's a gmo substrate, glyphosate or other herbicides/pesticides contaminate the product and change the biology of the items grown on them.
Anyhow... I don't sell Avi-Culture II....but I could if I thought I'd have enough customers. Anyhow, I recommend it based on it's sound manufacture and the fact that it is made for avian use.
You can tell them where you heard about it. He appreciates knowing that I recommend them.
http://www.avi-culture.com/shop.html
And...the website is pretty bad

P) But the product is good. The small container lasts a long time depending on the number of birds you have.