Exhorbitant Data charges

I have been using a Vodacom 3G modem for a year now without problems. I checked my available data on Saturday and still had 8 GB. Then apparently somehow on Monday 25/01/2015 I managed to use R 4,163.88 worth of data over and above the data I had available.<br> <br> I have never exceeded my 10GB cap since I got the device, so please explain to me how I do this in 1 night. <br> <br> Somehow on the same day, I managed to use an additional 4GB on my phone. (on which I upgraded the contracts that very day).<br> <br> Please can someone get back to me as how to resolve this issue as soon as possible