Over Charge on Credit Card

I have a credit card that has a limit of R6000. My April statement had a closing balance of R3585.38 and my May opening balance was is R7322.12. i called to enquire how this was possible because no transactions happened between that day. I was told that i made a withdrawal on the 17 April of R4000 that exceeded my limit. i then asked what was my available balance before the withdrawal and they say it was R2300 so i asked them how dose the bank allow me to withdraw R4000 when i only have R2300 it should say insuffcient funds?? I was then told that the credit card allows a person to go over limit then i asked by how much because i want to go and withdraw R100 now. I was told i cant because i have withdrawn over my limit of R6000. NOWHERE IN MY CONTRACT DOES IT STATE THAT I CAN USE R500 OR R1000 OR ANY AMOUNT ABOVE R6000 BECAUSE THEN I WILL KNOW WHAT MY LIMIT IS. I AM NOT PREPAIRED TO PAY ANYTHING ABOVE THE LIMIT THAT THIS CREDIT CARD HAS IF THESE PEOPLE SAY A CREDIT CARD ALLOWS YOU TO WITHDRAW OVER YOUR LIMIT THEN THEY MUST SAY BY HOW MUCH. AND YET NOW THEY SAY I CANT BECAUSE I HAVE WENT OVER MY LIMIT