One of my young girls passed lastnight. And from the feel of her little body and the symptoms she was showing I'm assuming it was Ascites. I'm going to thouroughly clean the coop and hoping it's not contagious.
From what I have read on here it is not contagious, more a symptom of liver failure or something. Some people have had chickens that had ascites for quite a long time. I wouldn't worry too much about it. Sorry about your hen! If you are brave you could open her up and see what was up.