Were you paying interest on your account or were you paying the balance in full each month? If you were paying just a portion of the statement balance (such as the minimum due) and were being charged interest, you have residual (ghost interest) due.
Interest on credit card accounts are accrued on a daily basis. Interest for the current statement starts the first of each month and ends the last day of the month. If the account balance due is paid the 15th of the month, the following month you’ll be charged residual interest for interest charged from the 1st of month until the 15th of the month.