I have a very obvious perianal fistula which can be seen but that did not show up on CT Scan or MRI, even with the "special" MRI to search for it. This is not uncommon, so do not doubt yourself based on these tests.
I certainly wouldn't wish a fistula upon you and would be happy for you if it turned out to be something else. But your symptoms do sound like a recto-vaginal fistula, and you are right to press for answers.
37yr female, dx UC '04, dx changed Crohn's Colitis '17
Currently: Stelara, Apriso, Prednisone *weaning*
Previously used: Remicade, Humira, Imuran, 6MP, Asacol, Lialda, Delzicol, Pentasa, Cortenema, Cortifoam, Rowasa, Canasa, Entocort, Uceris, Uceris foam
I'm Gluten-free. Supps: VSL#3, Cal/Mag, Vit D, Krill oil, EZFE