Vodacom overcharging and cannot give feedback

Vodacom cannot explain why they are charging me for data when I still have data available. According to their own log in I had 4.96G available out of 9G. My contract gives me 3G a month. They are charging me more than R1200 for data used, but they cannot explain how this is possible. I contacted them since the 14th of April, but they debited my account at the end of the month and still cannot explain. Real poor service Vodacom