I've had that happen once. We were practicing our falls in class on a pretty crowded mat and I got hit in the head by someone's foot as they were breakfalling. I saw stars for a couple of minutes.
I'm going to ditto everyone else and say that you're nages also need to be more aware while throwing you.
I really don't think the solution is to leave the dojo. Accidents happen, but not like this. Don't be afraid to say something to your sensei or a sempai that you trust that you think people should be more aware.