Massive increase in account

I've just spent more than half hour on the phone with 3 different consultants trying to get an answer as to why my cellphone bill went from on average R600 for the last 2 years to over R3000! All the consults could tell me was that it was my data usuage and when I queried this, they coldn't give me any details. they have logged a call with their billings department and said that it could take 3 to 5 days to get a response. I'm just trying to understand how this is possible when my usage was exactly the same as previous month. MTN please contact me. Its a good thing that i have already decided to cancel by contract and go with another provider