I used to practice Taekwondo before, my hips would pop when I externally rotate my hips after sitting for long and I assumed it was normal. No pain or snapping, just a pop like you might hear when you stretch yourself out after being a position for long. Now, I have starting working on my flexibility again. I have found the same popping has resumed after I started stretching hip flexors. Is this normal?
Go see a physiotherapist. Right away.
My wife, a dance teacher, showed the same symptoms and it turned out to be torn tendons. It needed to be corrected with minor surgery. Hopefully this is not your problem, but definitely get it looked at before it turns into something serious. If you're lucky, you just need a cortisone injection.