It could be a syatic nerve, (idk how to spell it) but it pinches a nerve and it shoots crazy pains down your leg. My mom had them all the time, still does. And she is miserable the entire time. Its common during pregnancy because of the extra weigh and the baby might be sitting on the nerve as well.
If could be that the baby is sitting on a nerve. If it's going down your back starting in the lower back and shooting down your leg, could be your sciatic nerve, which a lot of pregnant people deal with. If you're really concerned don't hesitate to get checked out. Could be anything....
Good Luck...