Hen has a red and inflamed butt

[alula]

About 2 weeks ago I noticed that my Black Maran hen had a red and inflamed butt with the feathers looking very sparse. I have examined her closely and at first she had a limp that was barely noticeable and it has gone away since then. She acts completely normally, eating and drinking as well as dust baths. Her rear is squishy as if its filled with liquid but I do not seem to feel an egg and I doubt she is egg-bound. When I feel her behind to examine her she seems to tense up. I have multiple hens and im unsure of her current egg laying rate but im putting her in her own area as of today to monitor that. She seems to be laying down alot more than normal and has that inflamed (almost balloon-like) rear but other than that she is fine. What should I do? I included some pictures below of her behind but I also included pictures of her feet because she was born with an abnormality and im unsure whether her butt has some genetic cause?

 

[Stravager]

I feel like I've commented this a lot lately, but it might be water belly? Have you checked her for lice and mites?
Someone else with more experience please comment and get this post rolling.