Do you have any pictures??? It could possibly be Vent Gleet (its a chicken yeast infection) My hen had this and I treated her and she got better. Here is what you do.
# Separated her
# Flushed her system by putting 1/4 cup of molasses in a gallon of water and let her drink it for 6 hours. (you will only do this once because it takes the good and bad nutrients out of her immune system) After 6 hours put her back with the others.
# Build it back up by giving her a Tbs. of Greek Yogurt or plain yogurt a day (Which gave her good Bacteria)
# Boiled eggs in her feed every so often gave her good protein
After I flushed her system I put her back with the others. I did the yogurt for about two weeks and she was almost back normal. I continued this treatment until she was all better.