High interest and bad attitude

Truworths account was handed over for collection original debt R8847.81, I received a call on June to make arrangements to pay the account, creditworx added their interest the debt was close to R10.000 arrangement made was to pay R1000 a month until May, each an every month after I have made a payment they always send me sms to check my balance, end of March 2015 after they have debited my balance was R2200, they debited on April no sms was sent to me for balance, this month I am calling them to confirm as I will be finishing to pay the debt on 27/05/2015 as per initial agreement, First agent (Ongezwa) told me balance is R2700 why, she could not answer, referred me to Wallet Isaacs and gave me a number that was not working, second agent(Lerato) told me I must speak to administrator , this morning I called them spoke to Grace Bronkhorst he told me interest was added, how can they add interest of R1700 in 1 month. I received a statement they are billing me on incorrect number they contacted, I fail to understand and interest shows it was added every month but all of a sudden they added R1700 that's RIDICULOUS.