Hard, immovable lump on the right side of the back of the head (occipital)

Hi, I have a rock hard lump on the back of my head, on the right of the occipital bone. I have had it for a few days now. It appeared on the day after I had a dental x-ray - not sure if this caused it but it's very sudden and in my 24 years I have not experienced anything similar.

I am waiting 2 weeks before going to the doctors. I have no symptoms so other than the rock hard lump and some swelling, I'm not bothered by it physically.

Has anyone experienced similar?