Vodacom bills me R3456 for data - not in contract

Last month, my blackberry 9780 was water damaged and had to be repaired, I did not have a phone for a few days and borrowed a Q5 to use while repair. Went into Vodacom Northgate, explained, did a sim swop for a micro sim to use Q5. My bill that averages R600 per month for years suddenly is R3456. I phone in & am told to wait for my statement on 03Feb as the lady is unsure of what the problem is. Nothing comes. I phone again, I email. No response. Today I call to cancel and am told that use of the Q5 incurs additional data charges. How is this possible? Is a contract not a contract? Should I not have been told this by the consultant in the shop? Howcome I have never experienced such bad service or negligence through Nashua? Vodacom has really taken advantage of this situation and I am horrified at their non-assistance until i want to cancel. How does a CONTRACT bill change so much in a month when it was hardly used? I cannot believe how I have been taken for a ridiculous ride with Vodacom and in my opinion this non-informing of a customer is actually a **** on their part.