I work at a vet hospital and to be honest the bleeding could be from anything. She could be bleeding from her vagina and not her rectum which would mean she's most likely in heat and that can only be fixed by getting her spayed. More than likely that's the issue but in all realities it could be anything. I've seen many animals come in for bleeding out the back end.
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Disregard the part about the heat, I didn't read the part that she's already been spayed lol