Deducting more than what was agreed in the contrac

I opened a new contract with Cell C last month and the payment amount each month on the contract was R236 a month. Cell C took the debit amount off today but they took R275 off. No where did i go over my limits on data or calls etc and my contract was put on as a Top Up so if i ran out of any data etc i would need to replenish it myself and not get charged the additional expenses for going over the limit. This is unacceptable.