ACCOUNT OVERDRAWN

I have a cheque account with Nedbank, and my overdrawn limit it R50.00 but this month they called me and and advised it R1245.16. I still don't understand how it reached that limit if there was no money in my account , how was I able to swipe then. I'm so confused and I don't understand and It means that next time I will be owing a million rand and they will want their money back. l'm not and I want t change my account to another bank, I only started banking with them this year in February , how come my account it not updated when I do not have money.