my little guy is sick

So my kitty Tiger has been throwing up since tuesday morning. We took him to the vet and the vet gave him some predigested food to give him (we couldn't feed him his regular food or water until wed night). Well Tiger hasn't touched the predigested food (and i don't blame him ). So last night we gave him a tiny bit of regular food and he threw it up. So we left out some more of the predigested food and he's still throwing up. My FH took him to the vet this morning and they're going to keep him overnight.

Has anyone's cat done this? I mean i know they throw up occassionally for whatever reason, but we don't know why Tiger is throwing up. If he had a virus or whatever, wouldn't it have passed by now (its been 2 days)? What could be wrong with him???