I have a concussion and was in the car with someone who had to decelerate quickly from 60 to 10 mph by slamming on breaks. could this hurt my brain??

Any concussion -: -is defined as loss of consciousness, & this does injure the brain, but it usually recovers. But repeated such injuries is of concern. If you were not unconscious & are doing well,you should be OK. Sometimes there's a thing called post concussive syndrome where head aches are common, but it is usually after a severe blow to the head.