Overdraft decrese facility is not user friendly

I recieved a lump sum of money which I wanted to use to pay apart of my overdraft owing, over the weekend, I tried to decrease my overdraft on my internet banking and the instructions on how to do this is so not user friendly, I tried calling FNB but they were closed, I have now mistakely decreased my overdraft to 0, when I called on Monday to increase it I was told that I can only do this in a months time, my money is gone now, and I have no money left for my debit orders running today. I have a good credit record, so I dont understand why I have to wait a month to increase it, as I paid most of it all already. very dissatisfied