Here is some reading about egg peritonitis and ascites with a photo showing where to drain fluid:
https://www.backyardchickens.com/t/853726/ascites-in-desperate-need-of-some-help-and-guidance
http://scoopfromthecoop.nutrenaworld.com/tag/laying-issues/...
It sounds like she may be suffering from internal laying, egg yolk peritonitis, or ascites (fluid in the belly.) How old is she? Has she been laying eggs lately? If her lower belly is full and tight with fluid, you could try to withdraw some of the fluid with an 18 gauge needle to help her...