I'm definitely not an expert, but I have had several (4-5) fistulae since 2006. Mine have all been confined to my perianal area, with at least one infiltrating my vagina. Personally, my fistulae always started with an abscess that I could feel that was really tender & painful, swollen & kinda hot to the touch & I would run low-grade fevers; when I finally looked at it with a mirror, it looked almost like an inflamed cyst. However, I never had an actual fistula form on its own. Every time I noticed the swelling, etc I called my colorectal surgeon & he did a pouch endoscopy & actually drained the abscess & created the fistula, which he usually then treated with a seton, which is basically some kind of rubbery string that they loop through the abscess/fistula area then through your anus & tie it off. It's then left in for months to years in my experience, the idea being to provide drainage for the abscess so that additional fistulae don't develop.
Having said that, in the decade or so since I've been dealing with them, I don't think setons are the best course because now I have 3 active, permanent (unless I can find another surgeon to try to fix them) holes that my stool actively leaks from. So, even though I have rectal continence, since my stool is so watery ALL the time, I'm forever leaking stool. I have chronic UTIs & perianal irritation, even when my pouch is healthy. I think my surgeon, after discovering I have Crohn's & not UC, basically gave up on my pouch as he continually advised me to convert to a permanent ostomy. However, I'm stubborn & loved my pouch compared to the ostomy. I was completely miserable during the months I had the ileostomy after emergency colectomy. My skin is super sensitive & I could never get a good seal, so I'm hanging onto my pouch as long as I can.
As for the diagnosis, mine usually started with a doctor's visit with a digital rectal exam, then my surgeon would do a pouch endoscopy. I don't remember mine stinging though, but mine tended to start outside of my vagina. Stinging I usually associate with irritation. Depending on the consistency of your stools, you might also have a UTI or other vaginal infection, as it's really hard to maintain hygiene with watery stool. Also, if you're taking any antibiotics like Flagyl or Cipro, that really ups your chances of getting a yeast infection, which can also cause painful irritation.
I hopehope someone more knowledgeable comes along to help you. For what it's worth, my opinion would be to see your GI doctor or pouch surgeon, especially if it doesn't get better or gets worse. The way my surgeon explained it, early treatment of abscesses is important because if they rupture it could result in fecal matter getting into your body which could lead to septic shock if it's servere enough. When my colon perforated I almost died because of septic shock, so it's nothing to play around with. On that note, it might be helpful to monitor your body temperature, since fever is a sign of infection too. Although, keep in mind that low-grade fevers are pretty common with Crohn's--not as sure with UC.
I hope some of this helps & you get to feeling better. Also, since I'm not sure of your diagnoses, if you have UC abscesses & fistulae are supposed to be super rare. In fact, my surgeon changed my diagnosis as soon as he determined that I did have an abscess because it's supposed to be so rare. Then again, if you have a j-pouch I believe that can increase the odds of abscesses/fistulae too, but I've forgotten a lot of my UC knowledge & research in the decade after my Crohn's diagnosis.
