Bank of America Credit Card Complaint

Bank of America Credit card department,

APR or interest rate Florida

I paid my credit card balance of $970.00 on -/-/16 due within the due date of the statement cycle and was charged interest of $4.00 on -/-/16. They stated that the interest was charged because i had a previous statement cycle that was not paid off. Why would I get charged interest for paying the balance due for that month. My understanding is that this is not allowed under Regulation Z. Thanks