Hi Val,
definitely finish out the antibiotics. In fact, I would ask to take them up to and after the surgery. (Flagyl is an antibiotic that lends itself to doing this, unlike erythromvacin or penicillin). I've found that drs. are relly idiots when it comes to dealing with these things. You definitely do not want this fistula/abscess to grow and track. If poo got caught there before and caused an infection it could very well do so again.