Change of Custody, change or orders, moving and Child Support

Complicated situation:
I have a daughter who is 16. Her mother and I were never married. I am very involved in her life. We live in different States. I moved to Nevada from Oregon. I paid child support in State of Oregon per a child support order last modified in 2006 until my daughter decided to come live with me in Nevada. At that time I requested that child support be stopped and at that time I requested no child support from her mother (which I regret). This was granted by the State of Oregon. My daughter decided she wanted to move back with her mother after a year. Her mother at this point has moved to Washington. Her mother at this point filed for Support in the State of Washington. I received a letter from the State of Washington mimicking the order from Oregon from 2006. I am confused how the State of Washington can use that order if I have documents from Oregon saying that the order had been dropped. We have no formal custody arrangements order from any court. Just child support. I would think that Washington would need to initiate a new court order regarding required Child support. All I got was a letter saying that I owe 2 months back support and the same amount that was from Oregon in 2006. Does anyone have any insight into this?