Well, if you don't know for sure how recently, could certainly be ascites (basically fluid retention) caused by infection (egg yolk peritonitis). I'm dealing with a Buff Orp right now who was laying very recently, but went into her first molt, which is when some egg issues can happen, and now...
Has she been laying at all lately? Is she a hatchery hen? Her age is when they begin with egg yolk peritonitis and/or internal laying. It usually shows up around 2-3 years of age, especially in the most common hatchery breeds.