What happens if I forgot to pay?

Amex normally had my payment due date on the 1st, but this last time, it charged me on October 31, and I forgot to pay. I realized today and made the payment immediately, but it now shows that I didn’t pay the previous period. This has never happened to me before. How much interest will I have to pay? Will they charge interest on the total debt or just on the proportion I owed for that month?

If necessary, I have the Platinum credit card.