Ridiculous interest

Good day

I have a Homechoice account. Although the account is in arrears I made and arrangement to pay a monthly fee of R600 a month, I'm now being charged a ridiculous interest amount. Last month when I phoned them to ask what my balance is, it was R2500. I made a payment in August in the amount of R600 which means the balance should be more or less R2000 - R2300. When I phoned in now the amount is R3600. How is this possible to jump from R2000 to R3600 in less than a month?