There is a bump on the right side of the back of the head. It is hard not moveable. The bump is under the scalp. Ita about the size of a nickel. Its on my boyfriends head he said its been there for years. He thinks he got it after he got hit in the head. Its never cause pain till recently he says he now gets really painful headache from that area. The pain eases when he puts some pressure on bump. What can this be?