Day light *******

On 27 June 2015 made a purchase via pos from one of the consumer stores and I knew for a fact that I had funds available for the said transaction and even got a sms confirmation that the purchase went through but 4 days later as my funds had ran out from my cheque account received another sms stating that my transaction had actually gone into overdraft as on the 4th day there were no funds available. Where on earth is this possible? Why am I being inconvenienced as the said consultant blames it on the store's banking... How is a pos relating to banking as funds should be put aside for the 3rd party to access as and when.... Fuming as this is not the 1st time this is happening