Swelling or growth on chickens butt?

[Lastknight]

This hen is about 5 years old, I have not seen an egg from her for several months now. She had some feces stuck on her butt feathers for many weeks, today there was some yellow poo on there. I decided to give her a warm bath after smelling a stinkyness coming from her butt. After I cleaned her off I found this lump as big around as a nickel and sticking out about half an inch or so. It feels fleshy, not hard. I spray some vetericin on there. I have been feeding her water with apple cider vinegar and food with diatomaceous earth in it for about 3 weeks to clear up the poo sticking to her butt, so far it hasn't helped but I found this.
Also, I have noticed some hard breathing from her for about a month or two now and she lays down in the grass a lot. She doesn't seem lethargic or sick. She will jump up and run around and eats regular. She had a prolapse about 2 years ago that lasted for a few weeks but it has not been a problem since.

 

[Tookie]

Not sure what the lump is, but to me all her symptoms sounds like ascites, or a reproduction issue.
Does she waddle at all when walking or running? Does her whole abdomen feel bloated at all?