Hmm... Let me think. I know this one. Before I got surgery for my abscess it had a really bad odour as well. I made a thread about it, and I'm almost positive the responses said its fairly normal for it to smell. When my nurses come to pack my abscess wound, they always smell it and say its good that there's no smell. I think its okay for it to smell, because mine has for years before. Don't quote me though.
Does your fistula cause you pain? Have you had a seton or anything else for it?