See a doctor. You need to see a doctor immediately.
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.