Its your syatic nerve. With your hips preparing for child birth there is a lot of pressure on that nerve. If you sleep on your right side weither you know it your not you could be pinching it. If lucky like all good things once baby is delivered this will go away.:-)
Mines my right one. And it's all the time. I hate it!?!!!
That's happened to me a couple times when I wake up. Not sure why though. It's kind of sporadic.