Charged for no reason

Charged for no reason, they claim I had no funds when the transaction happened. This has been happening often lately and its really frutrating, i received payment and decided to purchase online and money was deducted from the account but few days later when i checked account, i found that i was charged R200 for no funds item, this also happend last month. i talked to one of the agent and told me that i get charged after the purchase has happed and if i dont have enough funds for the purchase that has happed , there will be a fee charged, i dont understand since money was deducted already but i get cahrged for purchase that has happened already, this is a **** cos it looks like i keep paying for charges on the item i already bought and on top of that i pay monthly fee of R120. this lady ended up hanging up the phone she could give me proper answer. i moved to FNB because Nedbank was doing the same and i thought this will be better