Hello I Just Went Too Clean My Cats Litter Box

Hello I just went too clean my cats litter box and I noticed a bloody stool. Problem is I have 2 cats. So I don't have any idea which it could be. 1 a female about 10 years old. The other a male 3 1/2. The male had an urinary blockage due to crystals. I also had him neutered at the same time. This was about 3 months ago.