Hard lump on lower left side of skull? My 12 year old son has a hard lump/knot on his lower left side of his head (behind the ear).  It has been there for about a week, that we know of.  It was somewhat tender and sore to begin with.  This is under the skin not like a boil or something.  It is about the size of a pecan or maybe a little larger.  What could this be??  He was just recently tested in the past for swollen lymph nodes in his abdomen area as well, had a scare with that one.

Summarized question: What causes swollen lymph nodes on the lower left side of the skull, behind the ear?