Since it seems to be sudden onset, I'd suspect that it could be intestinal parasites, or that she ingested something that bothered her GI. Did she drink cow's milk or is she eating a new kind of food? Is this your pregnant cat?
A single bout of diarrhea doesn't need a vet visit, but if this persists for 24 hours, it's best that she sees a vet.
Diarrhea can cause dehydration, so be sure that she's drinking enough water. If she's not drinking, give her water through a syringe. You could also make an electrolyte drink, which will help more to prevent dehydration. There's a recipe for that here: Cats and Dogs Naturally: Electrolyte Solution Recipe