Why is my cat pooping out bloody stool?

He's a five year old male. He's been doing this for about 6 to 8 weeks. We tried to change his diet, and that worked for a while, but recently the blood came back. We're taking him to the vet tomorrow, but we're still worried. The blood comes at the end of the poop, and doesn't seem to be causing him any pain. What could cause this?