I have a small hard lump behind my right ear. I have headaches nausea, and dizziness. It doesn't hurt to the touch... What is it?

Likely a lymph node. There are a number of lymph nodes scattered around the back of the head and neck where they are easily found. At rest they may be the size of a pea and when stimulated they are often bean sized or larget.Size often changes over weeks & often reflects bug bits, skin rashes or throat irritation, it is not likely related to headaches, nausea or dizziness. Your doc can confirm this.