CHARGED INTEREST EVEN THOUGH BALANCE WAS SETTLED IN FULL!!!

I PAID OFF MY FULL CREDIT CARD BALANCE ON THE 30TH OF JUNE - NOT 1 SINGLE CENT OUTSTANDING!!! I HAD PREVIOUSLY PAID THE MINIMUM PAYMENT DUE ON THE 13TH OF JUNE - WAY BEFORE THE PAYMENT DUE DATE,OF 3RD JULY. EITHER WAY,MY FULL BALANCE WAS PAID BEFORE THE DUE DATE - WHY THE HELL THEN,HAVE I BEEN CHARGED INTEREST THIS MONTH??? IF YOU DON'T REVERSE THIS WRONGLY CHARGED INTEREST AMOUNT,I WILL TAKE MY BUSINESS ELSEWHERE! THIS IS UNACCEPTABLE!!!