Sometimes it is hard to know exactly what is causing swelling in the lower abdomen until a necropsy can be performed on the body after death. Symptoms may give a clue, such as if the hen is still laying eggs, having abnormal eggs, or has stopped. Is the lower belly full and firm, squishy or spongy? Egg yolk peritonitis or internal laying can cause ascites or water belly as a side effect. So can cancer or fatty liver disease. How old is the hen? Is her tail up or down? I have had hens who had swollen lower bellies and who lived a couple of years. I always open their bellies for a necropsy when I lose one. Taking pictures of the organs can help with a diagnosis.