I absolutely agree with molly. Passing blood could just be the symptom of severe gut inflammation, or it could be something more serious. Definitely not something to ignore. My cat started doing it a few years ago (no blood though) and I was very upset as I was pregnant at the time, wouldn't have been a good idea to have an incontinent cat, especially with a crawling baby later on. The vet reckoned it was an infection, and after antibiotics he gave her some "good" bacteria (bit like Yakult). It settled down, and no probs since.