My mom said she experienced something like that at night when she was pregnant with my sister, and it turned out her hip was popping out of joint, and she'd feel better when she got it back in place.
I experienced something like that at night when I was pregnant with my first, but it was because he was sitting on a nerve, so it got better if I changed position (but moving was even worse than doing nothing!).
I don't know if it could be either of those things with you, but they're possibilities anyway. Talk to your doctor and see what s/he says. All I can think of is to try changing positions when it happens; like I said, it hurt me worse when I moved, but after I moved there was no pain at all. I'm not recommending that you put yourself through crippling pain, by any means, but if your doctor doesn't think it's anything harmful, it might be worth a shot.
at 11:28 PM on Oct. 3, 2009