Diarrhoea in cats can be caused by changes of food, parasites, infections, inflammation of the intestine or other disease, among other things. Both the small and the large intestine can be involved, and the colour or consistency of the stool does not always give a reliable answer about where the problem lies.
Keep an eye on your cat's appetite, thirst, weight and general condition. Note how long the diarrhoea has lasted, whether your cat is vomiting at the same time and whether you see blood or mucus. Kittens and cats with other illnesses can be affected by fluid loss more quickly. If the problem keeps coming back, stool tests, blood tests or other investigations may be needed to find the cause.
Contact a vet if the diarrhoea does not clear up, or if your cat becomes lethargic, stops eating, loses weight, vomits repeatedly or seems dehydrated. Black stool or larger amounts of blood should be taken seriously.