Billed wrongfully

I have been buying a 1 gig bundle every month and all has been fine until November. As usual i bought my 1 gig data and it depleted mysteriously, i called to enquire and no one would assist me. I bought another 1 Gig on the 17th of December 2015 and it also depleted mysteriously with no accountability, i had to buy another on the 2nd of January i bought 500MB and it also depleted mysteriously, i called to complain and no one assisted me. I bought 20MB yesterday and this morning i recieved a text that my data is depleting and all my apps have stopped working. When i called i was told to check that my apps are restricted and not running on their own, i did and they are restricted, i was told to check google play and see if anything was running and as it turns out nothing was running further more i was asked to dial *141*5# to check if i have any subscriptions and as it turns out i dont. Then i was asked to call the billing department who only confirmed my claims and said i must check my settings and internet usage. This was no help at all. I use the same data every month and it lasts all of a sudden now my data disappears and MTN does not want to account for it they are blaming me.