Possible welfare fraud

I have been paying child support for over 17 years. Last year, September 2008, a hearing was scheduled for a hearing at the Domestic Relations (DR) office. It was determined that the custodial parent was working. In October, 2008, I recieved a letter from the IRS Offset where I owe TANF benefits. I immediately notified the state of PA, about my concerns and the possibility of welfare fraud, my case was immediately moved to Ohio, where I reside.
1) Why am I responsible for paying back the welfare?
2) What steps should I take