Bonus data payment

I have been on contract with vodacom for few years now.I pay R150.00 a month for 20gb data but suddenly I'm paying more for the bonus data that I never subscribed to.I cannot be paying an extra R250.00 for data that I am not even using.there was no communication sent on phone to say you'd be debiting my money how do you debit such a lot of money when I haven't even subscribed anything.I feel like vodacom is cheating here.I am not happy at all because last month more money was debited in my account even this month.I have a wifi already and I cannot afford spending so much money on a data that I have never subscribed to and I do not even use.please vodacom cancel this subscription,coz if not I am definitely porting to a network that would value me.