There are two main reasons why credit cards generate interest: First, if an overdraft is withdrawn, three periods of interest will be generated. Because the interest on overdraft withdrawal is settled on the bill date, the interest from the bill date to the repayment date is It will be included in the next period's bill, so the interest for three periods will be displayed. Second, if the loan is not repaid on time or is partially repaid, interest will not be waived. Since you said it was not overdue and you have never withdrawn cash, the only possibility is that you did not repay in full. If you really can't figure it out, call the customer service hotline and ask for an explanation. You can always find out clearly by combining it with your account details.