Potential Emergency: Is vomiting normal after getting a flu-shot?

My 7 month old boy got a flu shot today. After a feeding of his routine milk while playing he vomited it all. When should I be worried? Wife thinks he might have hit his head? (she read somewhere on the interweb that vomiting after awakening has something to do with the brain) My guess is that vomiting is a side effect from flu shots (this is his second flu shot in two weeks).