First a doctor would perform a vaginal exam to see if a fistula can be located. If one cannot be seen or felt, then the doctor might order a test like a Transvaginal Ultrasound.
Treatment of a rectovaginal fistula consists of either a medical or surgical approach depending on the problem that the RVF is causing. First, antibiotic regimes might be tried. Most commomly, Flagyl and Cipro are ordered to see if they promote healing. Remicade, an medication given intravenously, also is used for treating fistulae. Finally, surgery might be ordered if the medical treatments don't work or if the problems associated with the RVF warrant it.
You asked what causes fistulae. Unfortunately, fistulae are something that people with CD can have happen. A fistula in CD is a tract or tube that forms from the small bowel or colon to the outside of the body or to another organ. It is generally thought that the inflammation of CD weakens the intestinal mucosa causing it to bulge out. Pressure from the contents of the intestines pushes on the weakened area causing it to bulge out further and further until it attaches to the outside.
I agree with all those that have encouraged you to see your doctor about this problem. Infection is a concern when a fistula outlets in the vagina.