Internet usage

I am on a vodacom 2 gig contract. On 2016-03-04 i wanted to connect to the internet, A vodacom page informed me that the data bundle has been depleted. I cant understand because my usage on the internet is not above 2 gig a Mont, How ever i went and purchased another 2gig top up on my bundle. This was done during my luncah hour from home. When i want to connect with my laptop after lunch (30) minutes the same page came up that indicated the data that was purchased was depleted. Just to mention my laptop was not connected to the internet during that time, When i called the call center they just indicated that the data was used. the could not give me any explanation on how the data was used