DATA USAGE

I have a red contract with vodacom with an average bill of R1300.00 per month for more than one year. All of a sudden i get billed R3800 for July 2015 and R3900 for August. Phoning them does not resolve the matter at all. Being told by a employee of chatz Waverley that they receive a lot of complains regarding the above and that the data and updates should be off at all times to avoid additional charges for data usage, and that i am being charged R2 per mb that is being used out of data bundles.??? I am no computer expert but this means i pay R2000.00 for 1 gig.??? that is out of bundle. The concern is however that my data usage on average is the same as previous months with a bill of R1300 average. If the above is true then vodacom will loose me as a client for sure as i am not paying R2000 for a item that is worth what R150?. When and how do they notify there customers about this???.