I thought you might be on an SSRI. Prozac sometimes causes nausea and vomiting. It is a rare but known side-effect. Add a concussion and you could have a rough time. Ask your prescribing doctor if he/she has any ideas.

I was on Paxil for 14 years and occasionally had bouts with nausea, especially when in bed. They would hit me out of the blue with a vengeance then go away or linger very mildly. My stomach would do a few flips. That feeling of 'almost nausea' was frustrating.