Negative balance after card fee debited

I went to FNB to downgrade my cheque acc to savings account due to financial crisis- FNB normally collect la card monthly fee last day of the month - to my surprise my account is sitting on -R90 because they debited R100 monthly account feee there is R15 of service fee for what I don’t know.

Number 1 I don’t get paid on the 20th and I have never - what gives FNB any right to go into my account and start debiting - no one said anything about dates changing.

As a results I am not happy with FNB and they better fix this mess - where is card fee charged in the middle of the month and no consultant advised there will be a card fee debited during the month.

I refuse to pay FNB that is why I will be moving to another bank after 14 years of being with them.