Extreme overcharging for data

My contract with MTN came to an end in December 2015. However, I continued to receive 2 GB of data from them per month at R79.00 so I didn't bother cancelling it. The problem arose when I got a new computer and my Google Drive started uploading onto the new computer while I was using the MTN device as a hot spot (Note, I was not aware that Google Drive was uploading). When I saw that I had exceeded my data usage by just over 1GB. I expected to be penalized but I was shocked by the amount. MTN charged me R2, 521.59 for that extra data. This is despite me using less than 500MB (my average consumption is 399MB per month) of my monthly 2GB over the past 13 months. Please would somebody at MTN look into this as I feel that I have been cheated.