The "a" word is ascites syndrome - http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/204601.htm. It could also be blood or peritonitis. You could try draining it and seeing what color it is. There are a bunch of people here that have done it and prolonged the life of their chicken. It's worth a try.Maybe? I am not sure. It's so hard I can't really even feel around much... but I don't feel any solids or lumps. I have a feeling this is going a bad way. Now I am wondering about her being and internal layer or that other issue (starts with an "a") where the body cavity just fills with fluid.
Link on EYP - http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/205804.htm