Data disappearing

I loaded 500mb of data to my mtn ****le for my pc on Sunday 30 August for R105. On Monday I saw I had 20mb left. I thought maybe I typed in the wrong number and sent data to someone else so on Tuesday 1 Sept I loaded 500mb again for amother r105. I checked to see if it was loaded and it said I got 200mb free and had 700mb in total. Not even 10 min later it said my data was finished. Mtn kept asking didnt I use the interner too much or maybe my pc updated and took data. I have been using the ****le for 3 years and know how it works. I have never used over a gig in 2 days. I know how my pc works. I asked how can mtn guarantee if I pay yet another r105 that my data won't disappear again. She could not answer me. R210 and over a gig data down the drain...