HUGE billing problem!

I have a huge bill each month and on request for further investigation they could not see the problem or assist me in the matter of my data usage. They how ever take a guess on what is taking up my data. I must say they did give me a once off discount on my bill, for only this month ( thank you for that), but the problem has not been solved yet. I don't use most of my data that I receive on my Vodacom contract that I pay for, on top of my huge bill each month so why is the amount of my bill so much..????? Is somebody else using my data? I don't know, nobody can give me any answers! If I do my own investigation the usage of data on my phone is not even a third of what they bill me for. So where is my data going and what is the problem that no one can solve?? I am really not happy with the service I am receiving at the moment. I believe I am paying for someone else's data usage for a while now. With an VIP unlimited contract it is still not possible to use over 5GB of data in only 6 days and have a bill of over R 2000 only for data usage. Is there anybody within Vodacom that can help?