I Have A Marble-sized Lump Protruding From The Back Of

I have a marble-sized lump protruding from the back of my skull on the lower right quandrant. This started on June 28 at twice this size, and I thought it was some kind of bug bite as I had been outside all evening the night before. The original swelling reduced by half in a few days, but has now stayed the same for 6 weeks. It is tender to touch, and is slightly soft at the base and harder at the top. What are the possibilities?. What type of doctor should I consult?