Diarrhea is not an uncommon problem with cats. Normally, it is caused by something as simple as a dietary change or an intestinal irritation. It can, however, also be caused by parasites, bacteria, or something as bad as pancreatitis or liver disease. If your cat has diarrhea for more than a couple of days, you should definitely take her to your veterinarian to help find and eliminate the cause! If you see blood or mucus, you should call your vet right away. If you see worms, your cat may need to be taken to the vet as well.
It could be something that he had to eat, or drink or whatever and stuff.
Sometimes it can be something minor, but sometimes it can be something more, so I guess the best thing to do is to call the vet or something to see if she or he can tell or give you something so you can give to the cat or do to him, or meaby you can take him, if it lasts too long.
She drinks my left over milk when i am finished with dinner.
Have you been giving him milk?