I have four indoor kitties who have occasionally had small amounts of blood in their stool. They are healthy, happy kitties-the vet checked a stool sample but didn't find anything. The blood is bright red, so it seems to be coming from near the end of the colon.
Some cats have a little blood in their stool occasionally, with no identifiable disease.
But if it's associated with diarrhea, occurs more than once a month or if you're sure more than one cat is involved, you may indeed have a problem. Large intestine parasites are the most common cause. So far, you're two for two. The most common large intestinal parasite in cats is a protozoa called giardia. It takes a special stool test to diagnose this organism.