Next time you have a doctor's appointment ask him to feel the bump and let him tell you. Remember - the skull is shaped differently on everyone so you may have a bone which sticks alittle further out on the back of head.
If the lump/bony bump is indeed a permanent fixture near the base of the skull, it's most likely the classic "External Occipital Protuberance". If, however, this is a new growth on the skull, or if...