FNB messing with my account and trying to milk money

Yesterday morning i had a debit order that went off it goes off on the first but if it falls on a weekend it usually goes off the Monday night,so i wasn't expecting it to go off, anyways i got a payment notification from fnb and my account was in a minus by R550, i then transferred R600 from my savings pocket for this debit order and then another 100 plus to buy some stuff at pick n pay, last i checked last night my account had a balance of R5, this morning the debit order came back returned, but yesterday i cleared it by transferring money into the account for this debit order, I fail to understand the logic behind this? how did it come back returned when i transferred the money to clear this debit order? What do i do now because i dont have the money available for this debit order anymore? How does FNB just mess with a persons account like this? Not just that, theres cases where i swipe my last money R20 eg. and then it says reserved for purchase this gets taken out of my available balance even if it says reserved and im not able to use this money, but then i get charged R 125 because they pay for something else with that money and now they charge me for my account going into a a minus because i dont have a clue what they doing? Just trying to milk the last bit of money out of a person?