What should I do about my best friend??

Me and my friend Judy have been best friends since we where 5 years old we are both 25 right now. Well last night me,her,and another friend of ours where on our way to go see a movie. Well the car I drive is a Buick and its really bumby on the roads and she suggested that maybe its the shocks but we have had them looked at and they are fine. I told her a mechanic told my dad once that this kind of buick is bumpy and in the cold it gets worse. Well I turned the music up to a song I like a lot and looked back and saw her head wobbling all over the place so me and my other friend laughed and so did she. And she said Im telling something is really wrong with this car Ive never seen a car be like this before. I had to yell over the music once again that a mechanic told me this and she out of no where yelled at me and said Im sick of u being rude to me and hit me in the back of my head we have never hit eachother before ever. She knows I just had a brain surgery. What should I do?