Excessive Bills

I had a contract with Vodacom that ended December 2015. I chose not to upgrade my phone my continue with the monthly service. Since then I have received 2 bills - 1st one over R 2 000.00 and the second one this morning of R 1 640.00. I queried the one in January and was told it was due to high data usage. I could not understand that I used so much data as I have wifi at work and at home, but I made a point of switching off my mobile data and only use this when necessary. I cleared my data usage on the 1st of Feb so that I can track this and at yesterday morning I was up to 267 MB (my contract includes 300MB). All of a sudden it shot to 443 MB (with nothing running in the back) and I turned this OFF completely. Now I receive the bill of R 1 640.00? Why and how does that happen? Someone needs to call me to sort this out TODAY or we need to cancel this contract completely! Your choice.