I've been experiencing the same thing for a few years now. It's like it comes and goes where at first I'm normal, then my stomach swells like I'm 6 months pregnant and gets rock hard, makes it difficult to bend over etc,..
I went to a doctor a few years ago and they said "IBS", went to another doctor recently and they said the same thing "IBS" or gas retention (which I'm not gassy thankfully
) though like you, I don't see how this could be.
Keep us informed, I'd like to hear the results as well.