This sounds similar to what I’ve gone through with my Rhode Island Red named Cherry recently. Her abdomen and rear swelled up suddenly about six months ago. She seemed to be having a hard time laying, so I thought maybe egg bound. Tried epsom salt baths and probing and everything, no luck. Of course flared up over a weekend. Beginning of the next week I got her into the vet and learned it was egg yolk peritonitis. Essentially the eggs weren’t moving through her oviduct correctly, due to getting older, genetics, and inflammation. So the eggs drop inside in error, causing them to swell, can cause infection, and also fluid to build up as the body thinks the egg yolks are a foreign body. If not drained the fluid can put pressure on internal organs and cause lack of oxygen. My girls comb and waddles showed this with slight purple sections, not bright red like usual. Vet was able to drain and provide anti-inflammatory to help with oviduct so that perhaps she could pass eggs normally and also provided a course of antibiotics. She got much better for quite some time, especially over the winter when her body was taking a natural break. It’s been flaring up again in the past few weeks and we actually just got home from a vet visit for another fluid drain today. All the best to you and your girl.