Overcharge on data

I was due for an upgrade on my contract and myself and my fiance went in to get a new phone. We then got an upgrade on a mtn topup. They then told us that the new contract will kick in when the old one finishes somewhere in october. Today i receive an email for an amount of over R1000. They can review my account and i always average around 400 rand. I never go over my data limits and i still had data left in May. I always have data left so how can they charge me. I will take this further and i do have 4 contracts with autopage and il cancel it 1 by 1 and i wont stop there. I will not pay for this.