MTN Deducting airtime from me?

@ MTN South Africa has been deducting airtime from me for the past 24 hours. I do have any subscriptions. I called the agent, she said the reason why my airtime was a lot after the calls I made is because MTN had given me a bonus? A bonus how when I had rechraged two times? I then asked if a 3-4 minutes call would charge R 35 worth of airtime? I made another 47 seconds call and they charged around R3,83.

My balance was therefore R56 in the morning yesterday. When I checked my balance around 8pm it was R33. How is this possible?