A fistula typically is the result of an abscess forming under the skin, and the infection it causes "tunnels" its way outwards. If you have an anal fistula, you will have a hole close to the anus which will discharge pus and blood. If the discharge you are having is still coming out the "normal" route, it may not be a fistula, but could possibly be an abscess or infection (if you have a tear inside, the pus would exit from it into your bowel). Other signs of an abscess in that area would be painful lumps under the skin, and pain sitting down.
Definitely run it past your GI on Tuesday. My GP (not my usual one) misdiagnosed what would have been a treatable abscess a few months ago, and i've just had to have surgery to treat the fistula it eventually became.
Abscesses and fistulae aren't dangerous so much as troublesome, but as with most things, the earlier they're diagnosed and treated, the better.
Hope this helps. Good luck with your appointment!