Interest charge on my account

I received a sms a while back stating that Game won't charge me any interest on my account, but for the last couple of months interest has been charged. Example, on the 10/10/2015 my credit available on my account was R1634.49, my account was debited on the 02/11/2015 with an amount of R1440.15, so the credit available should now be R3074.64, but the credit available is only R1197.98, where is all the interest going. R3074.64 - R1197.98 = R1876.66, I cannot believe that you can R1876.66 interest. This is really ridiculous. This is only an example of the last 2 months, what about the other months where interest has been charged. This needs to be sorted out as I am very upset and cannot wait for this account to be paid off so that I can close my account with Game, but with the way it is going now with all the interest charge it is going to take me for ever to settle this account.