Well, a rectal prolapse or hemorrhoid could be there, as well as something more serious like intestinal bleeding. Hemorrhoids usually pass on their own, but rectal prolapses require some additional care. Intestinal bleeding would not have a cure, unfortunately, and would be the cause of serious GI upset caused by infection/bacterial imbalances (clostridium, runny stool), or overuse/misuse of some nonsteroidal antiinflammatory drugs (NSAIDs), like Metacam (meloxicam) or ibuprofen.
It's probably best to have the vet determine where it is coming from. Are her poos normal? Is the blood on the poo, or just the litter?