This happens to my son as well on occassion. He has only every spit up/vomited 2-3 times but I've seen 'stuff' in his mouth more often than that. It sounds as if it is acid reflux but, if he's not crying or fussing (and his doctor doesn't think it's a big deal), I would say don't worry about it. (although it makes you wonder if it is acid reflux why he's not crying, right? At least, I wonder about that. :?)