My (almost) 11 week old kitten has diarrhea.. Manx breed.?

I own two kittens, both of the Manx variety. One a dimpy rumple and one a stumpy. They are brothers from the same litter. I made a vet appointment yesterday for their big set of shots, they go in one week from tomorrow, on their 12 week mark. The rumpy, Liam, has had diarrhea the last two days. I think it's probably just because he got into my room mates older cat's food dish. I now lift that cat's dish up if my two are in the area. He is still playing, eating, and drinking PLENTY of water. He's bright-eyed and alert and doesn't make any noises or show any sign of pain when he eliminates, or when I scoop him up. I'm pretty much just trying to get some reassurance here that muh lil' one is okay.