I get weird sudden groin/thigh cramps too. The other day I really confused my husband when we were talking and I made a face at the cramp, he thought it was at what he said. The only shooting down the back of the leg sensation I've had was definitely sciatica, a little chiro and some stretching helped a ton. It could be just the baby pressing on a nerve or something?
Mommy to DS1
July '09 and DS2
Oct '12 and someone new in May '15