Antibiotics CAN help fistulas and fistula drainage. of course it depends on how complex the fistula is, and the type/cause of fistula. i have heard of some fistulas healing with antibiotics, and in the case of crohn's fistulas, antibiotics, remicade and humira biologics can help. i have a tiny rv fistula and have been taking antibiotics for a couple months and i do notice a difference. i don't think it is common, but it is possible for rv fistula to partially/fully heal on antibiotics/meds. a lot of fistulas do require surgery, but there are some that heal with conservative treatment. my doc said that antibiotics are the first line of treatment for fistulas, unless your fistula is really big, then they would probably operate. fistulas are often a result of inflammation/infection so the antibiotics help with that so the fistula can shrink, or at least so the area is not infected. the tissue should be healthy and free of infection, ideally, before they do fistula surgery anyway.
it is also possible you may have had vaginosis or inflammation in the pouch/vaginal area that was healed with antibiotic use. in some cases, fistula may be very hard to find. but if your symptoms are manageable and/or treatable, and they don't think you have one, that is great news! maybe it was so tiny, it healed!