Vodacom data disappearing

I've just upgraded to get 5GB every month, but more than 2Gb disappears in first or second day of the month. I only use the wifi for occasional private email checks and Internet (no downloading of videos, etc), so don't expect to use up 5GB in a month. This month I did not use the wifi modem that much at the beginning of the month, but when I checked the balance this morning around 10:00, I had+2,3GB. I logged on to just check a few emails (without downloading anything), then logged off. When I tried logging in again after 12:00, I received a message that I only have 5KB. i called Vodacom but they couldn't explain what happened to my data, and if I want them to investigate I should write a letter and send ID and pay +R20 to get a report. This is not a once off event, it happens every month and Vodacom doesn't seem to care or assist in getting to the bottom of the problem. The only option is to terminate contract. Can't be paying for data that dissapears on its own.