It could be intestinal parasites such as worms. Have you put any flea spray or anything on it?
Vomiting and Diarrhea
When a kitten has diarrhea or vomiting, your first priority is to take him to the vet. The loss of body fluids at a rapid rate will dehydrate your kitten quickly and he could die. Both of these symptoms can be part of a larger health concern and a vet should be sought promptly for guidance and proper medication. If your kitten's bottom becomes sore and red, a small amount of olive oil on the cleaned area will soothe it without an adverse reaction, or your vet will prescribe an ointment called Panalog. You can give your kitten some canned pumpkin in their formula or food to help make formed stool. If there is blood in the stool, this is always a sign of a health issue. Coccidia and Giardia are sadly common illnesses in health compromised kittens.http://www.kitten-
http://www.kitten-rescue.com/vomitting_diarrhea_and_constipation.htmll