Didn't read all the replies BUT
I had a hen with a swollen backside, sort of underneath and a little bit between her legs.
I took her to the Vet, (yup the Vet) and he gave me Lasix. After the 10 day treatment she was still the same and he said to me, she is a $10.00 chicken. Meaning how much do I want to spend on a $10 chicken, I got the drift.
A friend did the deed for me and he cut her open and she was full of growths the size of large marbles and golf balls. So I presume it was cancer.
A friend had a chicken with the same symptoms. She is a ER Nurse so she drained off the abdomen and the hen is fine.
I have a hen now with the same symptoms and as long as she is eating and drinking and not in pain (that I can tell) I will let her live her life but when she is looking under distress and in pain. My friend will do the deed for me.