Unknown deduction on my credit card

I've had a credit card for a few months with capitec, I've been paying them as one should.

For some reason when I transfered some money in to the account the other day it just went missing, not on any statement. All I could see was the money in. When called they told me it was because Capitec allows you to go over your limit and then charges you extra. I received an sms stating I was over my limit and they would deduct the money with the installment at the end of the month. Not one person in the branch or phone could tell me how I went over my limit that I signed for. And not one person could tell me why my money was taken then and not at the end of the month like the sms stated.