This is one of the reasons more and more Docs are requiring payment of co-pays/co-insurances-deductibles at the time of service.

OP, (as gigirle already said) you knew you received a service and would owe a fee. Espeically given the situation of your recent move/change of address, you could have been responsible enough to contact the Doctor's office and confirm the amount you owed and send in the payment.