I've got a little lump towards the back of my head, I found it a few years ago and freaked out. I instantly rang my best friend, turns out she has one too. Since then I have been to the doctors to double check they said it is absolutley normal, load of people have them, they can remove them if they get too big, but generally they just leave them as they are small and not at all dangerous.
I have no idea what causes them, my best mate is a fitness instructor and is really healthy and I have quite a healthy diet too, so I doubt it is diet related.