Yeah, fistulas generally begin where this infection or fluid build up. The difference between a draining abscess and a fistula, is that the fistula is lined with epithelium (similar to skin). It implies something that has been there a while, since it takes time to develop. A fistula is more of a problem because the body senses it as something that belongs there, so it does not heal up like a wound, or an abscess.
If the fistula is not associated with an infection or the anastomosis site (where the pouch joins the anus), there is the concern this is related to a Crohn's diagnosis. Perianal/perirectal disease also is associated with Crohn's, but these complications can also be just due to the surgery and possibly reduced blood flow because of the surgery.
In the meantime while this is being sorted out, hot sitz baths are your best bet. They improve circulation to hasten healing, plus they keep the exit wound open, so they can heal from the inside out. This is the only way for them to heal without new abscesses forming.
Hopefully, your doctor will explain this fully when you ask him to.
Jan