credit card over billed

My agreed installment for my nedbank credit card is R250 pm , today I woke up to an amount of R750 debited from my account. When I called nedbank I was told that due to little amounts of transactions I went over my limit. Nobody advised me of any changes or notifications received. I did not get any statement that shows I am over the limit. I feel cheated and **** to. I did not plan for this amount