10G of data depleted within 2 days and incorrect billing

Within two day 10G of date is depleted. This also happened last month. On the first of every month 10G of date is loaded as per contract and within two days its all gone, how? when I call the call centre no explanation Is given, I am sent from pillar to post. This happened in September and again now in October. Date used to last until the 20th-30th of the month but recently it only lasts for two days. There is nothing new that I do or browse with my phone. Now I am left with no date for the rest of the month, this is totally unacceptable and I am not happy at all. If need be I will cancel this damn contact as its costing me much more than what I had anticipated as I now need to top up on data. And the extra 88cents that are debited why as initial contract is R140 p/m