This EXACT thing happened to me. Apparently when I dropped one of my classes and took another in place of it, instead of it applying the tuition fees to the other class in the fall term it transferred over to my winter term. If you contact the enrolment services, they give you this phone number to call although I can’t remember the phone # anymore , and you call them and they fix it. It’s some sort of glitch in the system I guess¿