Has her voice changed at all? It is normal sounding or weak/raspy? One of my chickens had a lump in her throat and her voice went all raspy. It turned out to be canker (trichnomasis) which is treatable. Canker can cause weight loss because it's harder for them to eat and lesions can occur in the crop. If you take a flashlight and look in her mouth can you see any yellow cheesy lesions? The lump could also just be part of her trachea, you can feel if the other chickens have the same bumpiness.
I've also experienced gradual weight loss with my flock that a fecal test proved was coccidiosis. After I treated them for it I fed them a grower feed for a while (with calcium supplementation on the side for the layers). So I agree with the other posts above that if a fecal test is available it might be a good idea.
However, since she is suddenly 'skin and bones' after a gradual loss I'm wondering if she has a infection in her digestive system. I had a bird die after becoming 'suddenly' skinny and I thought it was mycotoxicosis (because she had gotten into a moldy bag of corn a month earlier).... unfortunately, after necropsy it was confirmed she died from a severe bacterial infection in her abdomen that eventually spread. It would have been treatable with the right antibiotics, I felt so bad. She also had a very strong sweet smell to her from her mouth and also her bottom (feces). She acted very normally aside from the weakness and loss of condition. She was still eating but not getting any nutrition - that is was caused her weight to drop so rapidly. Her droppings became watery and green/yellow. Her feathers looked bad.
I've also experienced gradual weight loss with my flock that a fecal test proved was coccidiosis. After I treated them for it I fed them a grower feed for a while (with calcium supplementation on the side for the layers). So I agree with the other posts above that if a fecal test is available it might be a good idea.
However, since she is suddenly 'skin and bones' after a gradual loss I'm wondering if she has a infection in her digestive system. I had a bird die after becoming 'suddenly' skinny and I thought it was mycotoxicosis (because she had gotten into a moldy bag of corn a month earlier).... unfortunately, after necropsy it was confirmed she died from a severe bacterial infection in her abdomen that eventually spread. It would have been treatable with the right antibiotics, I felt so bad. She also had a very strong sweet smell to her from her mouth and also her bottom (feces). She acted very normally aside from the weakness and loss of condition. She was still eating but not getting any nutrition - that is was caused her weight to drop so rapidly. Her droppings became watery and green/yellow. Her feathers looked bad.