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.
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