Data usage over charged

I called the call Centre on the 21 March 2015 complaining about my debit order for R1,309.91, I was told that it was for internet usage and that something was changed. I was debited again for April an amount of R1,353.67, after using Wi-Fi from work during working hours and switching my data off for most of the day? I now need a detailed statement reflecting when I went on the internet and for how long I was on the internet and now being billed for +/- the same amount as last month? I feel that I am not being treated fair because MTN has not contacted me regarding this CHANGE to even explained to me what is going on my account? Since I did my top-up I have never been billed this much and someone needs to inform me why since Dec the amount can change so much.<br> <br> <br>