With a pop, my water didn't break?

Kali
So 39 weeks, and showing lots of progress, losing my plug, I can feel my hips separating... 
I'm not sure what just happened, but layingin bed this morning I felt a 'pop' and a strange sensation near my belly button. Maybe I even heard it? I could be imagining it I guess... I fully expected water to run rapidly under my bum. I even tightened my belly muscles thinking maybe that would push some out.. Then.. Nothing? 
Is it possible that I felt the sack removing from the walls of my uterus? Like maybe it should have popped, but was just lucky it held together?? 
What was that??!