Charged interest on credit card I had fully paid within 31 days

It has been a month after getting my ABSA credit card. This is what you say about your credit card "We offer up to 57 days interest-free on qualifying credit card purchases. What this means is that you can spend on your credit card and repay what you owe within the interest-free period without incurring interest'

This morning you deducted the interest from my account for the month i have used the credit card. I had paid back the money within 31 days and that does not exceed 57 days.

Please explain why or I have to cancel this credit card!