A possible growth on my skull?

So, I have this hard, round lump on my head, about the size of a marble, on the right side close to my ear, it feels like it's attached to my skull and the skin has no problem rolling over it, it doesn't hurt or anything, but I'm just wondering what it is. Actually, I did have headaches for the past two weeks non-stop, I don't know if that's an important detail, but I highly doubt that caused it, I probably didn't notice it before, it could've been there for months for all I know. Anyone have any idea as to what it might be? I'm a little concerned, but I'm sure it's nothing serious.