I did not receive a bill last month. The current bill shows a past due amount and charges interest for the charges from the prior month. What should I do?
You should let your card issuer know. While you can call the card issuer, in order to protect your rights you must send a written notice to the card issuer.
You should follow the instructions on your statement. Note that the address to which you should send your written notice appears is usually different from the payment address.
Once you have given notice, you do not have to pay the interest charges in dispute while the card issuer is investigating. If you pay the undisputed amount, the card issuer cannot treat your payment as late and must assume that you have paid your balance in full when determining how much you owe next month.
If the card issuer determines that you are correct, the interest charges should be removed from your bill. If the card issuer determines that you are incorrect and that the earlier bill was sent, the card issuer must notify you of the reasons for its decision and let you know how much you owe and when your payment is due.