What Does Excessive Vomiting Indicate?

I have an 18 month old son who constantly vomits. Always at random times, no common variable that I can see. Vomits in bed, in car, on couch. Morning, afternoon and evening. Last time he had one of these episodes was just after his 1st birthday in February and it lasted 3 weeks vomiting daily. He recently started again vomiting the first time 15 minutes after his immunizations. A week later (Today) he has vomited three times today. He s perfectly happy in between puking episodes, I check his temperature every time as he has seizures accompany his fevers and he s a perfect temperature. I don t know what to do about this, I feel there is an underlying issue. After his last episode in February I had extensive blood work done for him and everything came back fine. I don t understand why he s always puking.