The first time I had a hen that had it she had trouble walking, couldn't sit down and seemed to be short of breath. It was when I first had chickens many years ago.
Now I usually find it before it gets that bad. If a hen looks off I feel their abdomen and backside. Also, I randomly feel up...
It sounds like Ascites aka water belly. She may have an internal infection. I have drained the fluid off of several hens. It will make them more comfortable but I eventually lose them. One survived for about 8 months before the infection got too bad, even with antibiotics.
I could be wrong...