My rectum is bleeding I think. Maybe vagina. How can I tell?

See a Dr. Rectal and/or vaginal bleeding can be very serious. The #2 killer in women next to heart disease is rectal cancer. Although this is rare depending on your age and family history it should be looked at. There are many causes of "vaginal" bleeding, most are benign, but a physician should evaluate...And differentiate between the two types of bleeding.
Put in tampon. Put in a tampon for a while. If it has blood on it when removed, the blood is from your vagina.