Should have it checked out ASAP. Is it a mole?
It is probably a harmless cyst. When I was 15 my bump on my head started out fairly small. Now it is huge. I went to the doctor just last week and she told me it was a harmless cyst but that she is going to refer me to a doctor to remove it because mine is off and on getting infected and it gets in the way and is becoming obvious. They told me the removal is simple and takes an hour or less, and you will have no bald spots. I just haven't had time to go back to the doctor to get the procedure done. You could go see your doctor, but I almost guarantee you it is the same.
It is likely a sebaceous cyst or lipoma, but there is one kind of brain tumor that causes lumps externally. You should have it looked at. Even if it's a sebaceous cyst, you might want to have it surgically excised because hair will stop growing over the cyst.