What Does Red Bump On Head Indicate?

I was in a car accident 5 months ago and hit my head hard on the back of the seat. The head rest was not on so my head hit the metal stcking out for the head rest. I went to the hospital and the ct scan came bace normal so they sent me on my way. I still have a tender red bump on my head that isn't going away. I want to know if i should be worried and what it might be. thank you.