Data disappeared

I have been calling Telkom for the past 2 days and I'm not satisfied with the answers I have been getting, I'm on a 40gigs data package which is 20gigs day and 20gigs night and for some reason I have about 8gigs day data left for the month of June. What i want to know is what happened to my 12gigs in a space of 3 days. I'm not getting a response that is satisfying from your agents. Can someone please explain what is happening with my data.