Personal Loan!

Hi, I have had the most disgraced experience with Standard Bank. I have a personal loan of which my installment was R1500.00. In March I made a revolving loan. I can't remember that i signed a paper saying my monthly installment will increase. And for the months following Standard Bank debited my account for the R1500.00 (so i didn't even made the payments myself). Since I've closed my account from which they debited my account and payed the installment myself. I have received numerous calls saying I'm in arrears and my installment is not R1500.00 its MORE!!!!! If its more since I've loan again why did you still debit my account for R1500.00 and why after requesting numerous time for signed letter where by I agreed to pay more is still not send to me??