Overcharged every month

I have a Cell C contract (with tablet). my monthly instalment is about R229 (500Mb ) or so, but i noticed on the statements i am charged additional internet usage (called smartdata internet which doubles my monthly bill so i end up paying over 500 rands per month for this which is not what i signed up for. Please note im not talking about the the time when i increase my limit. I am talking about what reflects on my statement as extra internet usage how can i be billed extra for internet as after my limit i have to wait till the next month to get an increase.<br> what hidden costs are there. i once called to enquire and got some stupid answer like certain websites get billed certain rates which are outside the scope of the contract so i asked which websites are those and i was given another feeble answer. so please contact me urgently cos i cant be paying these huge amounts every month.