Fistulas just don't happen for no reason. You have some underlying inflammation going on some where in your colon and you should see a GI and ask for an colonoscopy to see if you have crohns .
I had two massive rectal fistulas that appeared an inch from my anos and tracked back to my rectum, and during surgery to install setons, the surgeon did a colonoscopy and took samples. This is when I was diagnosed with CD in the lower colon and that was 2007.
GI wanted me to go on remicade as he said fistulas as large as I had would never fully heal by themselves. Instead , after much research, I decided to follow the SCD diet and the only drug I took was pentassa. Within 6 months both fistulas were completely healed to the point where the setons had to be removed. My other symptoms of CD were gone also like blood and white muscus in my bms.
I still follow the SCD diet to this day and I have not had one symptom from my CD or another fistula. This diet is not easy, and it takes 100% dedication on your part for it to work correctly. I suggest reading the book "breaking the vicious cycle" which goes into great detail about this diet and how it works. You have nothing to loose by trying this diet and all to gain if it works for you.