When your kitty is miserable with a stomachache, vomiting or diarrhea, you're liable to wind up miserable, too. Intestinal problems are common in kittens because their developing digestive systems are sensitive to slight changes. On top of that, they're more susceptible to infections and illness than adults.
Your kitty has a sensitive stomach. Sudden changes in food or too many human snacks can throw his entire digestive system out of whack. Another cause of short-term intestinal problems is milk. Dairy is responsible for stomach upset and diarrhea in as kittens become lactose-intolerant shortly after weaning. Don't give your kitty milk or other foods intended for people. If you have to change his food, do it slowly, adding a little bit more of the new food in place of a bit of the old every few days until you're feeding only the new food. Don't rush it; the transition should take a week or two. Food sensitivities don't usually cause vomiting unless your kitten is allergic to a particular food, but they can cause mild to severe diarrhea. A kitten on formula can have bowel problems from being fed too much in one sitting or being fed formula that is too rich.
Intestinal parasites such as tapeworms, hookworms and roundworms are more serious when they occur in kittens than in adult cats. A small infestation of worms in a kitten can have lasting health consequences. An infestation of any size can keep your kitty from getting all the nutrients he needs, even if he's eating plenty of healthy food. The presence of parasites will stunt your kitty's growth and make him sickly. Diarrhea, loose stool, vomiting and a pot belly are signs of worm infestation.
Inflammatory bowel disease can cause your kitty severe intestinal problems. Other illnesses that can give him problems include bacterial infections, panleukopenia, reflux, viral infections, liver disease and pancreatitis. Your vet will need to run tests to figure out if your kitty has an illness, since the symptoms of the different illnesses that cause intestinal problems -- such as vomiting, diarrhea, reduced appetite and lethargy -- are usually similar.
Just like people, some kittens go with the flow and others get stressed out by any little change in their routine. Either way, stress can give your kitty an upset stomach. A mild case of diarrhea is usually the only symptom, though; so if he's acting sick, it's probably something other than stress. If there's been a change in your household such as a new pet or a furniture rearranging, or anything else that interrupts your cat's daily norm, it's possible that stress is giving him a little tummy ache.
No matter the cause, keeping your kitten dehydrated is a top concern with intestinal issues. If he is vomiting or has diarrhea, he's losing fluids fast -- much faster than an adult cat. In cases of mild diarrhea, if your kitty isn't acting sick otherwise, you can try skipping a meal or two to let his stomach settle down, then start him back on bland foods like roasted poultry and plain rice. Encourage him to drink as much water as possible. Vomiting and diarrhea that doesn't stop or is severe needs immediate medical treatment. Your kitty might need fluids to get hydrated and medicine to stop the vomiting and diarrhea. Your vet will choose a treatment based on the cause of the intestinal issues. For example, a bacterial infection will probably require antibiotics, while internal parasites will require a good deworming medication.