Excessive Data Cost on this months bill

I received my latest data invoice and to my horror it was R1485.00 ... which I totally disagree with. I wrote a number of emails to Vodacom for them to investigate as over the past few years my data bill has never been more than R500 - I am on a contract for R249 per month (for 3gigs data) and if I see Im running out, I usually buy extra data. I also completed a survey on Vodacoms website complaining of this excessive bill. To date I have had no reply. This is unacceptable - I am a pensioner and also unemployed - why would I run up a bill of this magnitude?????? A response to my query would be greatly appreciated.