VODACOM DATA USAGE ENQUIRY

Hi. I have an internet/data contract with Vodacom: 2GB MBB 24 month - Contract (R139 per month). My contact is due for upgrade 9 July 2016. Since I have my contract I never had any problems until now. Whenever I use the internet I constantly check my data bundle to make sure that there is enough data so that I don't exceed the 2GG MBB data available. Never once did I receive any notice that my data bundle was depleted and every time I used the internet it showed that still have efficient data left. Out of the blue I received my invoice due 1 April 2016 in an amount of R1562.06. (I saw this on the Vodacom site on 18 March 2016 and immediately took my modem out of the computer and I did not use the internet once since then). I asked at Vodacom and they only said I used the data en need to pay for it. Needless to say, I had to pay the full amount on 1 April 2016. Yesterday I receive my invoice due on 1 May 2016 and to my horror my account is R3408.88 for data usage. I want to know how this is possible and the only answer from my Tzaneen Branch is: \You used out of bundle data and you have to pay for it\". "