What is going on here and what do I do?

Hey everyone, I noticed something was going on with my chicken’s butt today but I don’t know what it is. Any ideas?

I gave her butt a soak in some warm water with epsom salts and tried to trim the feathers back to get a better look, but unfortunately my scissors were pretty dull, so hopefully you can see it from the pictures. I tried to massage it and it seems mostly fluid filled and squishy but there may be some hardness as well on the underside. Her vent looks clear, the area in question is a few inches below it. She is not lethargic and seems to be acting normally. Barb is a buff Orpington who is slightly over 4 years old and would love to be in tip top shape again soon. Thanks in advance.