I have not been in this boat, so I don't have anything to add from experience, but it could be that the pap smear
opened your cervix a bit and some blood had been building up in there. since your cycles are very uneven, it's possible that you had been building a lining, or were even about
ready to have a period anyway. remember a pap smear is scraped off your cervix, and the cervix is what keeps the uterus closed and keeps blood from flowing out. I have no idea if it could have affected your fistulas, sorry.
I would say 1. use a tampon as if you had your period then if there is any blood when you wipe, you'll know it's likely from the back. you'll also gauge how much is coming from your uterus. 2. try not to worry too much, those long weekends are difficult when you have something on your mind.
wishing you the best!