Car Ride Pains?!?

Why is it when I get in the car and we are drivin I get the worst pains EVER! I don't get it when I'm sitting up on the couch or anywhere else, but in the car I can barely take it! 
Anyone else get this, anyone know why? I'm 34 weeks and everyone in my family on my moms side was born early and I feel it's coming soon.