Most likely it is nothing serious as long as it is red, but it is a good idea to find out what is going on. She could have an intestinal parasite or virus, bowel infection, or it could even be a urinary infection and the blood from her urinary tract gets mixed in with the stool..could also be a protozoan parasite called Giardia, but that is more common in cats that are in shelters or outdoors with others. This could even be a food allergy, or that her stools are too hard.
I would suggest adding a teaspoon of plain yogurt to her daily food intake, to put some good bacteria in her colon and hopefully clear this up. If it does not get any better, the next step would be for your vet to run some tests on a feces sample to see if it can determine why this is happening. If she came from a shelter, request a test for Giardia. Another symptom of giardia would be that the bowel movements would be particularly stinky.
I hope this is helpful to you and that your girl is better soon! Please let me know how it goes!
Cat Care Expert
20+ years experience with Cat Care and behavior problems, 3 years Pet Rescue