Can a Parent Retract Their Past Stipulation to the Amount of Arrears

My question involves child support in the State of: WA

Support order filed in WA in 2004, neither ex or myself still live in WA. I lost my job in 2011 and up until that time I was paying her directly. She went to Domestic Relations for them to attach my unemployment. At that time she made a written statement that I was current in my payment until November 2011 and at that point they figured out arrears amount. I then went on SSDI, she got a lump sum for our son and payments for SSDI went to her and since it was more than child support ordered, I have written notice that arrears balance was $0 and case closed.

I then won custody of our son this past summer and I am attempting to file child support on her

Now she goes back to WA and said she made a mistake and she was just trying to nice to me and now she wants arrears because I never paid her from 2004 to 2006 and I owe her $13,998

Child support division said according to our records there is no arrears and case is closed

Now she is taking to OAH I guess a person that reviews and makes a ruling

What do you think will happen.

If there's a stipulated order from 2011 declaring that support is support is paid in full through the date of the order, the odds of the court revisiting that order four years later are vanishingly small.