Worried about Protrusion at the back of the Skull

I have a question reguarding something odd on the back of my skull. I seem to have a lump there, I dont know how long its been there. I think since I was 8 that I can remember but maybe longer. I didnt feel back there regularly. Im almost 20 now. It doesnt give me any pain and it doesnt grow. Its hard just like part of the skull except its about the size of a small marble. Is it normal for some people to have this or should I be concerned? Basically I cant lay my head perfectly flat because of this thing. Like I said though, it seems to be part of the skull as it rounded and not just a protrusion in the middle of no where. Just thought id ask.