Debit order before pay date!!!!

FNB has debited my personal loan debit order and I'm only getting paid next week. My agreement with FNB is the last day of month not any day they feel like. I have visited my family for the long week, I'm stuck now financially, I no longer have money for petrol, food and to run my family errands!!!!!! What the hell am I suppose to do now????? I have to report or work Tuesday money??????????? The little I had until month end is gone. What kind of a bank are you? I want this debit order reversed immediately without affecting my credit record. When I say debit on a specific date you have no right to debit whenever you feel like it. I have a good credit record and I have never had a returned debit order.