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Hello. Thanks for the question.
That blood could be coming from one of a couple placed. It could be coming from the bladder or urinary tract. It could also be coming from the uterine stump where she was spayed. Lastly, it could be coming from the vaginal vault.
I would first start by getting a urinalysis done. I would do a cystocentesis where urine is removed from the bladder with a needle. This bypasses the vaginal vault, so we don't confound our results. If there is blood, then I would x-ray and look for stones. Otherwise, treat with an antibiotic assuming an infection.
If the urine is clear, then I would do a vaginal exam and look for a source of bleeding. It could be an infection (vaginitis) or a mass in the vagina. Mast cell tumors and other soft tissue tumors are not uncommonly found in the vaginal vault. If found, they can be removed and biopsied.
I hope this helps give you some ideas on how to move forward. Let me know if you have any other questions.