Incorret Billing

I phoned Nedbank contact centre on the 30 October 2015 and was given a balance f R3129. and my instalment is R1595 and payable on the 20th of each month. The loan is expiring in December 2015. In November I paid the normal instalment of R1595 and on the 02 December I received a message advising that I owe R1900. The consultant told me that is because they added interest. This is unbelievable as this does not tie up with the balance given in October. <br>