Go to the ER. Vomiting after a head injury may indicate something more serious like a bleed in the brain and is a medical emergency. If you have had a ct scan and there is not a bleed, then nausea and vomiting following a concussion can be treated with anti-vomiting medications such as Zofran or phenergan (promethazine).
Go to Emergency Room. Concussions are brain injuries. Repetitive vomiting is a worrisome symptom which needs to be evaluated quickly. After the emergency room has established it is caused by a concussion and not the more worrisome brain swelling (usually by checking a ct scan of the brain) medication can be given to control the vomiting.