Account to high

I use FB on my cel phone and buy data for this purpose. In the last 3 to 4 month I lost about 4 G because I was not able to use them all and it expired but still Mtn charge me for Data calls!!! this does not make sense... why am I loosing G that I have bought and then I get a use account for Data.. This is plain steeling. If I have some G available how on earth can they still keep on billing me??? Can you please look into this matter for me!!! Thank you..Acc Holder name. Mrs Gerbrecht M Jonker. Farm Middenspruit Suid Kroonstad.