Unacceptable Data Bill

I have been with Vodacom for a few years and have had good service, I have a data contract that is softlocked at 2 gigs and my bill has always been consistent until my last bill. I went to a customer service center and they called a lady at the call center. apparently i downloaded 3 gigs which I have no recollection of as I use my data for surfing the net. And for 3 gigs out of rate bundle I was charged an extra 1300.00 . How is this possible as I had systems in place to ensure that I dont exceed 2 gigs. I have always used my internet the same way and now all of a sudden a huge debit order goes of my account. The lady at the call center gave me a few excuses as to how i probably used 3 gigs, but i tried explaining to her that my sim is in a router and my router is switched off and there was no possible way of connecting to my pc or a cell phone. I would like to know who else I can talk to as I did not get much help from the call center. its frustrating when you have ensured that you have a softlock so as not to exceed usage the call centre agent says they are sorry as they do not know how it went over the limit.