Bleeding from any part or opening is an alarming sign which should be taken care immediately. If your cat bleeds other than heat period then it can be suffering from cancer. Cancer often develops from lumps that are an excessive growth of tissues. Take it to vet for a thorough check up. There is one more possibility that is your cat may have taken in hair that is irritating it and causing it to bleed.

 Is your cat fixed? If you haven't had her fixed, she could be heading into heat, and when a female cat has her menses, she is ready to mate. On the other hand if she is fixed, she could have something very serious going on that could be life threatening to her.
In that case I would get her to the vet as soon as you can. You don't want her to suffer if indeed she is suffering. Hope this helps, good luck.