From what you have written, it doesn't sound llike you have a fistula. Much more likely that the tissues of your anus are just really, really sore from over use. Have you tried topical ointments like anusol or prep H? What about soaking your bum in warm salty water (epsom salts)?
A fistula is a tract or tunnel that connects two things that wouldn't normally be connected that way. It can be a connection between two loops of bowel, between a loop of bowel and the skin or between a loop of bowel and an internal organ, like the bladder.
Often in people with periannal crohn's (where the crohn's is active near the anus) a fistula can form from the rectum through the perianal skin tissue to create openings near the anus, but outside of the bowel itself. Typically they start with an abscess that is very tender and hard to the touch. Often there is fever associated with it. When the abscess breaks, the fistula often leaks feces.
I have not had this kind of fistula, only bowel to bowel, so I suspect there will be others along who can explain it better, but unless there are things going on like i described, I suspect you are dealing with a bum that is just really sore from over use. I really find applications of topical ointments after every trip to the bathroom, along with frequent washing with warm water very helpful in controlling these awful symptoms.
30+ years living with Crohn's.