16yo girl. Two pebble sized lumps on back of neck, growing in size, and have been there for a week. Have been suffering dizziness, nausea, vomiting, and headaches. Any ideas??

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 larger.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.