is it on your skin or actually on your bone or what? it maybe cyst or something, easy to cut out if it is, but best get it checked none the less
Where on the skull? Might help us to know.
I dunno, but I find little lumps which come and go on my skull all the time, it's a part of being really really really smart... ;-] Maybe they're good ideas or something?
But seriously, if it doesn't hurt, there's probably nothing to worry about. Wait and see if it goes away, if it doesn't maybe see a doctor.