Data disappeared in a few minutes

I receive 500mb data on the 1st of every month. On December 2 I opened my emails and sent out 1 email. After that all my data was gone. I phoned 111 to query this, and was attended to by a lady called Karabo. She told me all she can see was that I had used the internet. She was going to refer the matter to another department which would investigate and call me. The call has never came and the matter of my data remains unresolved. This is not the first time data has just disappeared and Vodacom does not give a damn.