I have noticed a lump behind my 11 week olds ear today it stands out. any thoughts of what it could be. its only behind one ear.

Get it checked: If he is otherwise entirely well , it is probably a lymph node that may be reacting to a skin irritation in the scalp but there are other possibilities.

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.