My bank account was debited 4 months in advance

I have a contract with MTN for a laptop which covers data of 2 gigs a month. My monthly installments are 599,and if i go over the allocated 2 gigs, i am billed separately and the amount is added to my monthly bill. For the month of April, i was billed R3479.46 which makes absolutely no sense because I couldn't even use my data because my account was apparently \suspended". How can i be billed so much when I don't even know what what i am paying for? Even in my contract it was never stipulated that I would pay such an amount. It infuriates me so much because my livelihood is affected because of a stupid laptop of which i have never missed a payment for. I have been a loyal to MTN since 2007 and withstood its nonsensical service for too long"