My kitten is vomiting, has diarrhea, and won't eat or drink.

At 8 weeks your kitten can be on a completely solid diet, and if you're feeding regular cow's milk it could be what is causing the diarrhea (cats can't handle milk.) If she doesn't drink a lot and doesn't like moistened food, you can try adding in a pet fountain or other running water source that may encourage her to play or drink a bit more. If she is still having loose stools even after feeding only her regular food, a vet checkup to rule out parasites and GI upset is also a good idea. If she is trying to nurse your other cat, just separating her for a bit can help deter the behavior.

Given her age there are several things that cross my mind, but a common issue in these young kittens is parasites. Sometimes their parasite burden can be so high that they cannot maintain adequate glucose levels, and also can become uninterested in food and sometimes will begin vomiting. I would recommend deworming her. Also, it would be important to rule out signs of infection and/or sepsis. I think she needs to be seen by a veterinarian right away. Such a young and small animal can deteriorate quickly even with easy to resolve issues like gastrointestinal parasites.

Have you changed any of her food ? If so, it could just be the diet change as her tummy is still quite delicate and immature and it can take time and a few tummy upsets to get used to all the food ingredients. If no food has been changed, then I wonder if Pompom has been treated for worms. All kittens need to be treated for worms regularly, using a safe product dosed for her weight. You will have to take her to see a vet to get the right product for her. With regards to her food, just remember to change her diet gradually.

Poor girl. At her age, I would be most concerned about intestinal parasites or viral or bacterial infection. I would recommend a vet visit ASAP since she's have vomiting and diarrhea and could become dangerously dehydrated or hypoglycemia at her age.

Poor kitty. Vomiting and diarrhea in a such a young kitty can be caused by a bacterial or viral infection in most cases. If kitty gets into things it can also be causaed by a foreign body ingestion. I would highly recommend taking this fur ball to your local vet for a thorough check up and medications to stop the vomtiing and potentially antibiotics may be needed.. The fluids should help keep him from getting too dehydrated in the meantime. I hope that your little one is able to make a full and speedy recovery. Best wishes and take care.

It certainly sounds as though Zaytoune isn't feeling well! At 3.5 weeks old, we need to be sure kittens are eating every few hours to keep their blood glucose (sugar) levels adequate. If blood glucose becomes too low, animals can become lethargic and even begin to seizure in severe cases. Possible causes of vomiting could include gastrointestinal upset from his medication, diet change, a virus, etc. Since he is so young and not keeping milk or food down, I would recommend taking him to an emergency veterinarian as soon as possible to assure that his blood glucose levels are appropriate.

Withhold food for so long in such a small kitten is not recommended. Diarrhea is a signs referring to a intestinal problem. It could be due to parasitic infestation (very common in kittens) dietary indiscretion, inflammatory disease, bacterial diseases or viral diseases. You could provide bland food (boiled breast chicken or white fish or gastrointestinal commercial diet) for a 24-36 but, if she continues to have diarrhea, might get dehydrated and therefore need to be checked by a veterinarian. Also if you can spot blood in the faces or if she get lethargic and not eating, or if she start vomiting, I would suggest to have her checked by your veterinarian as soon as possible. 
Good luck and post again on the forum if you need further advices. I hope my answer was helpful.