Over charged for barely used SIM card

I have a 2GB sim card thru Mweb which has cost me R99 for the past 2-3 years and never more. All of a sudden in October an amount on R360 odd was deducted from my account. I called to query this and I was informed that I had gone over the 2GB cap in ONE session. I understood and accepted it as such. Now, end of November and again in December Mweb has taken upon themselves to deduct R498.00 exactly from my account. First of all, how is it possible that I used EXACTLY R399.00's worth data (which would be minus the R99 for the initial 2GB) for both October and November especially when they charge 99c per MB in out of bundle? Second of all, How is it possible to run up a bill so high when the device is only switched on once every 2 week - on average - for a simple 10 minute session on internet banking or facebook. I feel totally taken aback by this! Mweb believes they can deduct whatever amount suits them and when I asked for some kind of itemized billing they conveniently cannot provide it. I know I won't get my money back or anything like that and I'm sure when they call to explain it'll be the same ******* story. I have cancelled the ripoff SIM card and suggest that no one use Mweb!