Unexplained Data Usage

I took out a 3GB data bundle contract in July 2015. The first two months I ran out of data within 3 weeks which I thought was unusual - I thus had to top up 1 to 2 GB each time. After my third month I decided this could not continue, so I requested to be bumped up to the 5GB bundle. At the beginning of October I received my first 5GB and had a total of 8GB as 3GB were carried over from top up purchases. Because of these data issues I only use my modem when I need to, I do not leave it on permanently. I also need to add that I hardly surf the web and dont send and receive large files. My main data use is emails and internet banking. Yesterday around 18:00 I checked and still had 5.11 GB data left. At 19:32 I received an SMS warning me that I had less than 3MB data left!!!!! Please can someone explain how one uses that much data in 1.5 hours? I demand a refund of my data and a full investigation. If it cannot be sorted out then I want Vodacom to release me from my contract at no cost to myself. By the way, previous to this I had a 3GB data contract with MTN over two years and never once did I go over my limit. I can send all my MTN invoices to Vodacom as proof of this!