Mtn took my 3.4 gigs of paid data, overnight

Please explain how 3.4gig of data was taken from me overnight? When queried today at table bay mall, this is what I was told. I usually do 1gig a month, and because I had so much data decided to do 500mb. Low and behold because I loaded a lesser amount, Mtn took my paid for data. Please explain yourself!! How are we as consumers made aware of this? And what are you going to do about my lost data? I lost 4.3 gigs ..... I await your response with bated breath.