Paying way more than my original contract amount

I had an early upgrade on the 15 of Sep with MTN. The sales lady at the time let her manager speak to me because I told her I didn't want to upgrade before the actual time, it's just over a month and I will wait but the manager who spoke to me assured me that there will be no fees charged, I can upgrade early. She is an Indian lady from the MTN close to Fairlands and close to to the highway going to Pretoria. I upgraded and took no phone as I already took one with Cell C but upgraded to the 120 data plan on my previous portable rooter. Sep and Oct I was charged R433 and now this month they attempted to deduct a R1700 and now my bill is close to R2000. How in the world is that possible? I feel like I'm being ********** here. I also received an email to confirm my details. That can not be right if they already have all my details. Where an I going to get such a huge amount to pay MTN when I knew and promised I won't be paying anything extra and now this. The call centre people can only tell you what they see on the system and that's it. After this issue is resolved I don't think I will be staying with MTN again. I use to think they are the best but now I wonder.