egg bound or peritonitis? Need help to identify problem.

I have a hen who has a large orange sized lump below her vent. It is soft & squishy but she does not seem distressed or sick.
I think she might have egg peritonitis. But thought she would be sicker by now if that is the case.

I am going to proceed as if that were the case unless anyone else has another suggestion.

Does anyone have a picture of the underside of a hen with this condition I checked the site and couldn't find one. Trying to confirm this diagnosis.