Tightness, maybe cramping?

Question for anyone who's done this before. I'm 24 weeks. On vacation, been in the car 10 hours today. My back hurts and my belly kind of hurts too, I think from barely moving today and being in one position all day.

But most bothersome is that now that I'm finally lying down, I feel tightness down low and maybe some uterus cramping. Does that sound like that could be related to sitting in the car all day? Or is any cramping at all something I should worry about? No blood or any other weird stuff. I didn't want to call my OB before getting some outside opinions because I'm out of town, it's not like I could just run down there. Also it's Sunday and they're closed lol.