Charge for no reason??

My account ended in December. I upgraded to a new package smaller then my last one. I didn't receive an invoice in January so I overpaid, I received my February account excepting to see a credit and somehow Vodacom as managed to charge me an additional R760.00 for no reason?? why why why??? why????? I always pay this surely is your mistake, please fix it and rectify my account as to show my credit..... why!!!!!