The very first things I would consider are internal and external parasites. Check her carefully for lice or mites. Check the roost at night with a flashlight to see if there are mites visible. If she has not been on a regular deworming program then that could be the issue. With her weight loss and very pale coloring that would be my first suspicion. Other then that, very hard to determine. As Nambroth said, it could be many things. Egg laying/reproductive tract problems are common, as are cancers or other tumors and diseases. But, start with the most common and easily fixed possibilities for now, parasites, and work you way out from there.