Billing me for something I dont know

I have had a problem with vodacom because of the high charges and I decided to try cell c they had a bundle for 10 gigs + 10 gigs for R 299 per month this includes 10gigs + 10 gigs night time which amounts to 40 gigs I have used cell c for 2 months and every month they have charged me almost R 800 this is really ridiculous I thought i am gonna save money by changing to cell c but now they are charging me for something i dont know about. I used about 20 gigs a month. This really false adverti*****t