FNB does it again.Debit fees on 21st of the month!

I had funds in my account R507.20 as per 20/09/2015. On Monday morning I go to fuel up for R100.00 only to find that my card has been declined due to insufficient funds with a minus balance of R1371.23... I was shocked and could not pay for my fuel, which was firstly embarrassing and humiliating after the fact of knowing that I did have funds available in my account. I then get a message on my mobile stating that I need to pay R1871.23 into my account immediately. FNB ... since when do you debit bank charges on the 21st of the month??? I want my refund and any fees for unsuccessful debit orders back into my account and I will be changing my bank. It is my money that you took out without my consent... I want answers.