vodacom 3 gigs disappears before I use it

Two months in a row now, my Mygig data bundle has disappeared without me using it. This time (dec 1 2015) it disappeared literally within one day of it being loaded without me even touching it. I reported it twice and they just tell me that the data was used even though I know it wasn't! There is no point in them contacting me and telling me it shows as used, because this will not solve my problem. What recourse do I have? How do I prove I didnt use that data? They say that the moment it was loaded it was accessed and drained of the 3 gigs!This is my monthly budget. They are literally robbing my under my nose and telling me I can do nothing about it. I paid R180 for nothing! I didn't use a single bit of it. I have been surfing the net and see that there have been problems with many people mysteriously losing data. Is there a **** at Vodacom to force you to buy more data? And please Vodacom - DONT phone me and tell me it shows as used - you've already told me that - I maintain I did not touch it and it was gone within a day. I have had this contract for over a year and a half and it is NOT my device causing it, it has never happened before.