Incorrect Data Usage

I have a R69 for 1GB of day data contract with Cell C. I only use it to surf the web and have never used the full 1GB of data. Yesterday, I had about 350MB left when all of a sudden, it said that I had used it all and my current usage is R500 with my current session been 900MB! (in the time I was on, I probably used +-20 MB) I downloaded nothing, I have an old operating system that does not update and the only thing that does update is my free anti-virus program which updates +-10MB occasionally and yesterday it did not update. I phoned Cell C and was put through to 3 different people with the 3rd person saying that they can only be able to assist me at the beginning of next month as they need to see the invoice. How can an apparently large technology company have to wait to see an invoice to sort this problem out! Surely they should be able to see what happened on their system - Pathetic! So I have no internet for the rest of the month until this is sorted out. Imagine if this happened at the beginning of the month! Their only suggestion was to buy more data but I have no guarantee that this will not happen again. I only wish that I didn't just open this 2 year contract with them.