Please see a doctor as soon as you can. See a good colo-rectal surgeon who has experience with CD. I have a rectal fistula with a seton (a plastic ring that keeps the fistula open so it can drain). If it is an abcess, the sooner you get it drained and dealt with the better! If caught and drained early, an abcess may not form a fistula. As others have described above, an abcess forms because of a tear in the rectum or a gland in the retum becomes infected -- all because of the CD which causes your body to start attacking the lining of the rectum. The abcess can't drain so it forms a tube (fistula) to the skin to drain the infection. The fistula can end/empty out anywhere around the rectum, even going from the rectum to the vagina (if you have one, of course!). Once formed, rectal fistulas don't normally go away by themselves -- they need surgery in most cases. Your GI doc can recommend someone to see or go to the ccfa.org website and search for a doc in your area.
I was misdiagnosed and ended up with a huge infection and a massive case of cellulitus. I was on antibiotics for months. After the colo-rectal surgeon put in the seton drain, I improved very quickly! I am on remicade now and my fistula/seton doesn't bother me much at all. Really I went from near death to 100%. I feel great and am quite active! I'll get the surgery someday....
Let us know know how it goes for you! I'm praying that it all goes well and you heal quickly!