Fresh blood (often accompanied by mucus) in the stool are an indication of colitis (inflammation of the large bowel). Cats with colitis will often exhibit some straining with bowel movements and will have an increased frequency of defecation. The stool may be formed, soft or diarrhea.
The problem can happen for any number of reasons: intestinal parasites (a fecal exam would be a great idea), stress (boarding, vet visit, company, change in routine), ingestion of a non-food material causing irritation in the bowel on the way out, food allergy or dietary indiscretion (ate something she shouldn't have), secondary to pancreatic inflammation (less likely if she's otherwise fine), or an inflammatory or immune- mediated problem (immune system is misbehaving and attacking the body's own cells).
A bland diet (boiled chicken or a prescription diet through your vet) is recommended. You can ask your vet about getting her on medication (an antibiotic called Tylosin works very well for large bowel problems) to see if the problem resolves with a course of treatment. In most cases, this problem will be self- limiting and will clear up within a week's time. It's important to inform your vet if your cat is showing other signs (vomiting, lethargy, decreased appetite, fever, etc.), the stool is very watery, there is a large amount of blood present, or the blood doesn't resolve in 5- 7 days.
Hope this was helpful.

Edited by RGK on November 14 2006 at 3:35pm