bacterial infection
"I am taking the medicine for a bad infection in my tonsil. The first antibiotic I tried did not work.  On the 3rd-5th day of taking this, I developed a severe headache in the back of my head.  I was told this was from the infection and to administer heat, then cold compresses.  This relieved the pressure and headache immediately.  Thought this post might help someone in the future to know what to do.  I am feeling great today on day 6."