There are a few reasons why your kitten may start having diarrhea which are: 1. You rapidly changed its food. 2. Your kitten is stressed. 3. Your kitten caught the parasites. If it happens to your kitten: you should return to the food that your kitten used to eat before, and create a pleasant home environment for it. If the diarrhea isn’t going away after a few days, schedule a vet appointment and check your kitty’s poop for parasites.