More than likely, it is some kind of bacterial infection, based on what symptoms you have described. Really need more information to tell you more. I would isolate the bird, (always do this right away) provide normal food and clean water, and I would start a broad spectrum antibiotic like terramycin (it won't hurt even if it isn't needed). I can't narrow it down more than that with what you have listed for symptoms though. If you tell us more, we might be able to help. You have to really check her out, respiratory problems, thinness, color of face and comb, etc... Sorry I can't help more.