Just last week, we were really getting into it, I was on all fours pushing and grinding backward on his thrusting and I twisted and tucked my hips just right and tweaked an old injury.
Imagine your spine is a Rubix Cube. If you have ever played with one, you know you can twist the cube really fast, but sometimes in doing so, it can get stuck or feel like the pieces might come away from each other. Well, your vertebra are the same way, according to my PT who explained it to me. The first time I had this problem, 3 yrs ago, it felt exactly like that. This time was much the same (not nearly to the extreme as the last time), I was twisting, tucking and moving too fast and just tweaked my lower spine, near my tailbone.
All better now, but from what I have heard from friends of mine as well as my doctor, back pain caused by vigorous sex is very common.