Is it at the top of your butt and over to the side? Could be your sciatic nerve chucking a spaz. I know mine grabs me with severe pain every time I get up if I've been sitting or sleeping.
It's right through the middle/bottom of it, gets worse when she punches me too, though she keeps on turning around in circles as well (which makes it even more painful). Feels a bit different to my usual sciatic pain - that I can recognise pretty easily when it starts going down the inner and outer thigh, but this pain steers clear of the legs, just goes right through my butt and down to my v jay jay.