Incorrect Billing

I took a contract with the cell c on the 10th November 2013. My contract includes 100 minutes, 100 mb plus 300mb for free which equals to 400mb per month, 100 sms for R 149 per month. So on the 1st of November 2015 gives me usual benefits of 100 minutes, 400 mb, 100sms but the problem the have charged 300 mb for R 26 then on the 10th of November the took away the data they billed me for and they are saying I must pay full subscription on the end of the month. My problem is that I m paying full subscription for this month but they are taking some other benefits. 1. Why should I pay for something I didnt use? 2. My contract ends on the 10th of November 2015 so why should I pay for the whole month instead of pro data until the 10th November. 3. The time I took the contract no one inform me that I have to cancel the contract 30 days befeore the 10th of November, how will I know that I must do that.