My Airtime Balance Diminishes Without Me Even Using It??

Can MTN please explain to me how my airtime balance disapears without me even making calls? Last week my balance moved from R32 to R0 without me switching on my mobile data nor making any calls. TODAY, my phone was recharged with R55 I tried buying the all net minutes @ R10 p/day and it said I have insufficient airtime?? I literally made 2 phone calls at 1-2 minutes at most ( an i receive a discount every time i make a call) and my balance is now R37??

I am not happy with my airtime disappearing like this. Things are already so expensive, I cant afford to keep buying airtime that I don't even use.

Please help, please explain.