Could have been a constipated stool? Can also be from something in his diet, I have had a previous cat and also one I have now that both got very bloody stools from an ingredient in dry kibble....if on kibble try feeding cans for awhile, might save you lots on a vet bill. If it is medical they will likely sau IBD irritable bowel disease, But neglect to tell you the irritation is due to chemicals in the food
Hello and welcome...
Please let us know what your vet said.