Packet switched data calls

I have a MTN ****le with a 2 gig data bundle for about 3-4 years now. We have never needed to use more than that because we also have uncapped Internet at home. This month I was charged R8400 for Packet switched data calls. We never do any calls over the Internet and only browse things like Facebook, uploading some pictures. Yesterday I went to the MTN store and was told my balance was zero but today that massive total was deducted. How is it possibly conceivable that this 2 gig data bundle can amount to so much? I await the billing department to contact me but if this matter is not seen to soon I will be forced to seek legal advice, I would rather spend that money on lawers who would offer me a fair service.