It won't be a period. She doesn't have a uterus any more, and cats don't get periods anyway.
Are you sure it's from the cats? If you're the only person living in your home and you know *you* haven't been bleeding, look over both bats carefully for injuries. If you're not finding anything, take both kitties to the vet.
Hello~Just like with humans, any type of unknown blood should be checked. I would take her to your vet, it is better to be safe than sorry.
Have you checked the bottoms of the paws of each cat?