I think you are right about it being possibly egg yolk peritonitis or internal laying. It can be treated with penicillin and Baytril, but that may only give her time. It is a fairly common disease. Here are some links to read:
http://simplelifeainteasy.blogspot.com/2012/03/egg-yolk-peritonitis-diagnosing-sick.html
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx
http://simplelifeainteasy.blogspot.com/2012/03/egg-yolk-peritonitis-diagnosing-sick.html
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx