Kitten retches, nothing comes up?

I have two seven week old kittens. I know they might be too young to have left the mother, but I've heard that they are all right at this age. Last night around near midnight, Posha, one of the boys, went into his litter box (what a good BOY) and started crying, and then he started retching like cats do when they need to throw up. But nothing came up. I wanna say either he didn't WANT to throw up, he was dry heaving (he had just eaten so maybe not), or perhaps it didn't come up. My mom thinks it could be that he ate too fast/didn't chew a piece and maybe it got stuck. We grind Purina kitten chow in a coffee grinder (we cleaned it) and then moisten it with a little water. It's not a hairball, I'm almost positive. I'm worried and want to get their shots, but they can't have them for another week. The vet said not until 8 weeks. Finally, at 7:30 this morning, he retched like that again, but this time he look as if he was chewing something or trying to flick hair off his tongue. We heard that they can just be getting used to not nursing, eating too fast, or geting used to eating. Does anyone have any ideas? Does this sound like I should drive him in right away? he hasn't throw up again since (its 8:39 as of now) but i want to know so i dont panic if it happens again. thanks so much.