Delayed Transactions

Ever since I changed my account to a cheque I have started having problems. I am on unpaid maternity leave and have sent through a request for my car account to stop debiting for the meanwhile I'm not working but since their processes are taking long the debit order came through and bounced since I did not have enough on my bank account but what's astonishing now is finding that my bank balance shows that my account is in arrears by R1800 for things I swiped for while I still had money but went through only when there was no money on my bank account. How does that happen when as far as I know my account does not give provision for me to spend even when I don't have money. I am seriously confused and utterly disgusted by this standard bank account and need it checked out before I cancel this excuse of a cheque account.