Mtn
Overcharged not what was on contract

I upgraded my contract in December from a normal 8520 to a 9900 Blacberry on the same package. my account used to be below R500 per month and when i upgraded the package was -R150 more on the 9900. Eversince the account has been more than R1000 per month, this month R2200. I am doing nothing i have not done with my old phone, i even stoped using the 9900 3 weeks ago and went back to the 8520 and i received an account for R2200 not even going onto the internet. Please sort this out as no one can give me an answer as to what is the cause. And as usual i am stuck with the bill.