What Causes Recurring Vomiting Without Fever?

I have an 18 month old baby girl who has vomited several times. Probably 8-10 times in a weeks time. She is not running a fever and doesn t appear lethargic. She seems fine in between vomiting. She is eating, and not always vomits. She hit her head the other night and immediately vomited. and I took her to the hospital and they said she didn t have a concussion.