Irregular billing

I renewed a data contract in January 2015. My monthly bill averaged around R150 since then. I am careful with my device and always switched my data off when not in use. On 1 October, I got a shock when I was debited R4222,00! I immediately queried this with the MTN helpline only to be told that it will take around 20 days to investigate. A reference number REQ *** was provided. I also went to the MTN store where I took out this contract - they gave me the same story, but also told me to bring in the device for them to have a look at. What concerns me is that this was irregular and not flagged or queried as such. Agreed that it's my responsibility to monitor usage (which I do) - but surely there should be mechanisms in place to assist unsuspecting clients - I cannot imagine that I would have used as much data! I am now hugely out of pocket and would need to wait for some kind of response in 3 weeks' time - this is not fair! When I queried the possibility of changing the contract, I was told that a fee would be charged! I really feel **********!! Today, I had to phone in for a progress report only to be told that the billing is correct - no further explanations!!!