A *Pop*

Today I was in the shower and I wasn't doing anything out of the ordinary. Just standing under the water and all of a sudden I felt a *POP* inside my lower abdomin. It didn't hurt but it was slightly uncomfortable and I have no idea what it was. I don't believe it was my water because I think I would have felt the excess liquid coming down my legs besides the shower water. I'm almost 37 weeks btw, what could it have been??