Interest ****?

I have a revolving credit loan with Std Bank. Before paying in 49k to bring the balance to R0, I phoned in and asked if I would be charged interest. Seems these consultants are very unsure of themselves, but I was told interest would not be charged but I would be charged a service fee. I even went as far as asking if I had a credit balance in this loan would I receive interest and it was explained to that yes I would but it would be minimal (much like a credit card). Today when I checked my statement I see 2 interest amounts (R500+ and R200+); I phoned in explaining that because I brought the balance to zero middle of the month I expected to pay R500+ (normal monthly interest was +- R1000). I've now been told that I will be charged interest every month regardless if the balance is on R0. I understand being charged service fees for the facility, but to be charged interest on money that is not owed seems like theft. Can someone please explain this to me - If I were to keep this balance at zero, will I be charged interest every month? And how much?