Oh my goodness, the poor baby!
All the signs are pointing to a serious reproductive issue, possibly egg yolk peritonitis, salpingitis or cancer. Unfortunately, these issues are entirely too common with those poor abused high production battery hens. :hugs
The fluid in the abdomen is ascites...
I'm so sorry your hen isn't doing well.
How old is she?
When did she last lay an egg?
Does the swelling in her belly feel fluid filled or is it all hard like the lump you felt?
When you remove her from her soak, can you take and post some pictures of her vent and lower abdomen area?