Bad Service

I have a Credit Card with FNB and last month they told me my credit card was over its limit. I was fine with that. End of august they debited my cheque account with R3000 they said its the payment due. When I checked my balance on the 01 sept my straight available balance was R795 and budget available balance R3700. I swiped on budget 795 and withdrew from straight 600. Today I got a message saying my card is over limit with a R1000. How is that possible. Must my payment a month than amount to R4000. I need clarity and the account to be corrected immediately because how can my card be over when I have a debit order in place? I have to call them every month and these calls costs money.