Suggest Medication For Bump At Back Of Head

Hi! I have a quite big (like a thumb) bump on the back of my head. I have had it as long as I can remember, but Im not sure if it is normal. When I lay back in a bathtub, i cant rest my head quite well on the side of the thub, because the bumb is in the way. It is really hard also, like bone. Should I be worried? What can this be?
thanks