Naaiers

My agreement was that my debit orders goes of on the 27th of each month I am now sitting on a negative and need to give fnd over 300 for no reason just because some mothers ****ers tried debiting earlier how fair is this towards me I need to change my ****ing bank if this is the case.if fnb could explain to me why I need to give them over 300 because my account was not supposed to be debited I did not get paid yet