I noticed this about 4 or 5 years ago, but keep forgetting about it: there's a lump on the back of my head. it's under the skin and movable like a miniature knee cap. it's about the size of my thumbnail. it's not painful. should i be worried?

It has not increased in size in 5 years, and it does not bother you in terms of pain and there is no other growth in the vicinity suggests something that is of little concern and benign. The only thing is the cosmetics that may warrant evaluation for removal.

It is either a sebacious cyst or hyperplastic lymph node and both are benign, nothing to worry.