anyone can't lay on their back?

Re
I know your not supposed to sleep on your back which I try not to. It's when I'm not sleeping, and I lay on my back for a few minutes it really hurts to do so. I'll be 34 weeks tomorrow, and was just wondering if anybody else is having pain when they try to lay on their back? And if anyone knows why? Could it be because the baby laying on my back? Thank you