Overdrawn without credit

Why is my account with FNB always overdrawn. I did not sign up for an overdraft or credit yet my account every month is in a minus. I am so Angry. It shows I have money I spend then when I check again it's in a minus and I owe them. I also have accounts with Capitec and Tym Bank and never had this issue with them I can understand if my bank charges come off with no money but like my account is R560 in errors it's rediculice and if that's only bank charges I'll close my account. I'm really unhappy with FNB doing this to me every month when I did not consent to any credit. FNB Quote"How can we help you" Should be how can we ***** you over. I mean I'm on Maternity leave not getting my full income now my UIF money is gonna minus R560 and we already struggling financially at home. FNB - Worst National Bank