It could be worms or maybe just something she ate that disagreed with her. Deworming her might help to remove whatever might be making her ill. Also, cayenne seems to just perk up birds. All my birds get a dose of cayenne about once every two months and they love it!
From my experience, however, it could be any number of things. I normally don't diagnose the problem, but I treat thoroughly to prevent any problems. Oh, just thought of something else. Have you checked for mites? I've heard from others that they can cause some more severe changes in chicken's, including slowed egg laying. I've never had the problem, but it is probably worth looking into.
