I have always got carsick, my whole life. there's a pressure point behind your belly button which helps to stop it, and maybe bubs is pushing it the wrong way?!? (No medical foundation in this suggestion at all!)
Ginger really helps me with carsickness - on the way down to our wedding, a 7 hour drive, I ate about a cup of Japanese pickled ginger! Also, ginger beer helps. Anything ginger, really! I don't know if Vicks VapoRub is pregnancy safe, but I've heard that some people rub that around their belly button and that helps. What I used to sometimes do is put a bandaid over my belly button so it sucks in, and it makes a difference (yes, everyone pays me out and says it's a placebo, but a lot of people also say you grow out of carsickness, and at 27 I obviously haven't!) but that wouldn't work if you have an outie, and I don't know about a pg belly!

Good luck! I hope it gets better!