Child Support And SSD In Colorado

I was ordered by the court to pay $500 a month and did until I became disabled and on SSD. The mother didn't say anything about me not paying for 9 years because she knows the child isn't mine. The child and the man she lived with got a paternity test and she is his. Now CHS and AFDC are sending me letters stating they are going to withhold $600 out of my disability check. How do I fix this mess without paying over $50,000 in back support for a child that isn't mine?
Thank you.
Terry