i have a similar syptom (lump in lower/back part of the skull) without any hits lately. i'm not sure the reason either but will keep searching for answers and keep you posted. please do the same. thanks.
If it is at the base of your skull kinda connected to your neck it is a lymph node they should be one on the other side too. They tend to swell a tad if we have a bad headache or infection having to do with the neck or head. Mine swell when I hylight my hair just an allergic reaction. If it still bothers you give the doc a call and ask him.