I'm 20 years old, few years ago I hit the lower back right side of my head on a hard poll type surface. I have bump now but doesn't hurt. Damage brain?  

Doubt. If you had a brain damage then, it would have manifested itself by now. The bump you are feeling is most likely coming from the outer structures - skin, underlying fat tissue, fascia or perhaps, bone, but not the brain. A ct scan of the head or ultrasound might be helpful to figure this out.
You are OK. If there were not effects at the time, over these many years your brain has proven its usual resilience, and has suffered no ill effects. If there is a non-painful bump, you can probably ignore it but if it bothers you and affects things like combing your hair, then see your family doctor for diagnosis.