Astronomical bill charges

Over the past three months my data account has escalated astronomically! How does an account that used to be R600.00 a month +- jump to R5,000.00 per month. I have called on a few occasions to query exactly what has happened to my account to justify such an exorbitant increase. I am told it is an open line and my data is being used up because of downloads I am unaware of. My laptop has not changed in anyway for the last 8 years, so why now am I having sudden downloads. I was also told that the amount owing on my account is a roll over from month to month. I want to know why and who asked for this to happen. I have a debit order in place so why was there a roll over of outstanding amounts. I am truly and utterly disappointed with MTN whom I have been with for many many years.