Huge account despite limit on use of data

I was a Nashua mobile customer with a data contract.Nashua mobile clients specified a credit limit and received a sms once 80 % of the limit was reached. MTN sent me a sms on the 30th of April that my credit limit had been reached and that my service was suspended. When I phoned the MTN call centre on the first of May, because my service was still suspended on the first day of the new billing period, they indicated that my bill was R 648, I asked them to investigate as my account was suspended when I reached the credit limit of R 430. Reference number REQ***The consultant promised that I will be contacted within 7 working days. I went to a MTN shop and changed my contract to pay as you go. I received an account of R 993 from MTN. I phoned the call centre today as MTN did not contact me as promised. The agent could not tell me if any investigation had been done or why my bill is more then double the credit limit (actually put me through to a prerecorded message that my service has been suspended due to non-payment, my account is fully paid) . Please let me know why my bill is in excess of the credit limit and if my contract was successfully changed to pay as you go