Do you have a history of head injuries or where does your anxiety over hitting your head stem from? Based on what you said in your post, I really do not think just from sitting down in a car could cause enough force to be exerted to cause anything even close to a concussion. I do understand how the anxiety can kind of take over your mind with things like that though. Just this morning I dropped a very small light pillow on my head and I can't stop thinking about whether it could have potentially given me some kind of concussion even though logically I know that is just not possible. It kinda drives me crazy sometimes!

Just give it a few days and you will feel back to normal and probably won't even remember the incident. At least thats how it usually works for me, until something else happens where my brain won't shut up about whether that was a concussion or not!