What Causes A Hard Painful Lump At The Base Of Skull?

I have a small bony lump at the base of my skull (occipital) area. It is no larger than the size of a sunflower seed; but it is very painful to touch. It has been there for more than 6 months and has not grown; but as I said it is tender to the touch. I also noticed below it is another lump (not painful) that moves around under the skin.