blood in my cat's poop?

i noticed that since last night, there's a bit of blood in his poop and i don't know why. we don't feed him anything other than cat food. we give him a can of wet food and a handful of dry food. he doesn't eat anything else, at least none that i know of. he has all his shots and we gave the vet a stool sample a few weeks ago and the vet said he's as healthy as can be. we don't have any other pets except a turtle but the cat doesn't come into contact with the turtle. and he's an indoor cat. do you know what could be wrong?

Off to the vet sooner rather than later!! Blood in urine or stool needs to be checked out by the vet ASAP!!

Stephanie