Data misconception

I have been with Autopage Rink street and Vodacom for 15 years and I can not believe what terrible service I have had over my bill this month. My phone bill has stayed the same at an average of R750 a month for as long as I can remember, this month I recieved a sms saying my unbilled usage is an estimate R4818.97 excl.vat and monthly rental. When I went to see Autopage and got the data bundle in the first place I made very sure that the data was not able to ever run up and they informed me that it wouldn't,now they say there is nothing they can do and I owe the money for opening an email or update that took all the data. I am sure that it is not legal to not have a credit limit and when I spoke to their accounts department no one could explain to me how it happen and it was in the first 6 days of the month. I will be cancelling my account with vodacom when my contract expires in August. And I am still waiting for someone to explain to me how they can justify this. I have spoken to many people being caught out with this type of thing and it is disgusting that they can be allowed to do this. I hope that someone can give me the answer I need as I have been a loyal customer for so long