deductions without invoice

I didn't renew my cellphone contract and gave my notice for cancelling the contract. I however left my data contract active. 2GB per month for R99.00. End May,begin June an amount of R991.00 was deducted from my ABSA account from Autopage. Now, every month I check my Data balance and in the beginning of May I had about 4-5GB left on my bundle. I phoned the accounts department and the gentleman told me that I went over my Data limit for April - May, but how could I go over my limit if there was Data still transferred to the next month. My air time was running low and I told the gentleman that I need assistance in this. They never phoned me back, and I never received a revised invoice. how can they deduct money without sending out invoices? <br>