Cat peeing blood four hours after being released from hospital?

He went to the hospital four days ago because he was acting a bit crazy and not able to go to the bathroom. The vet kept him until about four hours ago and told me he had a build up of crystals that was blocking him or something like that but he was doing fine now. I THINK it's pee, although I suppose it could be something from one of the other cats. But if it was him (and he's usually the only one uses that litter box, that's why I assume it was) is it normal or should I take him back to the vet?