My question involves child support in the State of: Washington

My child was living with someone other than my X for 5 years. The X had legal custody accorting to the divorce decree. The amount of Child support is in Divorce decree. I payed my X child support but less than the full amount that the X agreed too. Now the X wants the full balance of all the money. Since the X did not have custody for 5 years what should I do. The X filed papers with the State asking me to pay $$. Are divorce paper a court order?