Contract cost has doubled

I have a 24 month contract with MTN, a promotion that they ran last year: A sony xperia z5, for +-R450/month. This included 2GB of data for free for the first 12 months, thereafter I will recieve only 500mb/month for the remaining 12 months.

The lady who assisted me with my contract explained at length that the 2GB is only for 12 months, thereafter I will only recieve 500gb, and if I wanted more, I would have to buy it separately. I was happy with those terms and agreed to take up the contract.

I app**** for this online and my handset was delivered to me. For the past 12 months my account cost R-+450/Month. Now this month it is +-R900. How can this be possible?

What has changed on my account? Why am I now paying double for my account? I did not upgrade anything on my account, this is not what I signed up for, and this is not acceptable.