Product: Credit card or prepaid card

I have a MasterCard with Bank of America. In XX/XX/2019, I paid the credit card off in full. I made no new purchases, but the next month they charged me " interest on purchases '' even though I had none. Since I'd paid it off in full, I didn't think I had any charges and I missed that they charged me interest " on purchases '' even though I had no purchases. I admit that I missed that I owed something ( even though I contend I did NOT owe anything - the bill I paid in full INCLUDED " interest on purchases '' ). I received an email telling me that I had a past-due bill and they reduced my credit limit from {$15000.00} to {$500.00}. I immediately paid the bill ( overpaid actually, because their billing is confusing ). 

Anyway, their charging me " interest on purchases '' even though I had no purchases, and then hitting my credit score for missed payment, and reducing my credit limit ( which also hits my credit score ) is an unfair practice. I knew what my total amount due was, and I paid it in full on or before the due date. Why would they continue to charge me interest on purchases when I'm making no purchases? And even in the bill where they charged me a late fee, they charged me MORE " interest on purchases '' even though I had no purchases that month either! They are charging interest on purchases when I'm making no purchases, and they continued to bill be for interest even though my credit card was paid in full! I am including the bill from them that shows I paid the amount due in full but there are still interest charges. I have the two subsequent bills that ALSO show interest on charges even though there are no charges, and I can provide those if you need. 

I initially contacted Bank of America and their representative could not explain the " interest on purchases '' except to say " the interest lags '' but other credit cards I've paid off do not have this practice - once I pay it off in full, they don't charge interest anymore. I've also asked them to reinstate my credit limit of {$15000.00} but they only said they would " take it under advisement. '' Please help!! 

I filed a complaint with the XXXX XXXX XXXX, and Bank of America sent the following reply : " Please refer to page two of your monthly billing statements in the section titled 'Paying Interest ' it states that we will not charge interest on Purchases on the next statement of you pay the New Balance Total in full by the Payment Due Date, and you had paid in fill by the previous Payment Due Date. We will begin charging interest in Balance Transfers and Cash Advances on the transaction date. Our research concludes that even though you had paid the previous balance for the XX/XX/2019 billing cycle in full, you did not pay the previous balance in full for the XX/XX/2019 billing cycle. Therefore, the account was subject to the applicable residual Interest Charge for the XX/XX/2019 billing cycle. '' I have never in my life heard of " residual interest '' - there is interest on charges and fees, and once those are paid, that's it. Imagine buying a car, and you call to find your payoff amount. You then pay it by the date they indicate, and suddenly they call you and say " it's not REALLY paid off, because we're charging you a mystery residual interest charge. '' It makes no sense to me, and appears completely deceptive. My card was paid in full on or before the due date, including interest on purchases. Period. Why would they continue to charge interest when I made no additional purchases? And then because I made a reasonable assumption that my card was paid in full ( which it was ) I missed that they'd charged " residual interest '' and charged me late fees, additional interest, and reduced my credit limit from {$15000.00} to {$500.00}.