Account

when I swipe for purchases a sms notification will be sent that I have swiped for example R50 at Shoprite and now my account balance is R50 meaning I had only R100 in my account on that particular date e.g 26/03/2015 then on 03/04/2015 my account will be on a negative with that same transaction I have made on 26/03/2015 which I was notified by sms that the amount has been debited now I am confused how swiping transaction works as it always leaves me at a negative balance which doesn't reflect well on me. I am asking that this thing be sorted I would understand if I swipe a transaction and no money is taken out of my account, something is not right and I am asking that it be fixed on my account, cause i understand that when the sms is sent the money is put on reserve so why does it reflect negative. As now I am scared to swipe for purchases as it will be deducted from my account and also put me on a negative.