bad service

Yesterday I called vodacom to add 1G of data on my account every month. Yesterday I received 1G Data on my phone and I only used it for facebook via Opera mini. All my application are switched off except opera mini and I always make it point that I switch of my data after using facebook but to my surprise today im only left with 543.93 MB and I only used 56.7Mb yesterday and today I only used 15.78. I called the customer care to find out about the matter but the lady who was assisting me hang up the phone I didn't get her name but it was around 13:07, I called again I was assisted by Tidimalo and she did the same thing she also hang up the phone on my ear. I really need to know what happened to my data because in total I used 72.48 Mb I cant be left with 543.93 and im not happy with the service I received when I called customer care.