If you were not current at the time your child support was stopped, then the interception of your refund would be to be applied to your arrearage. You would have received notice from the taxing authorities that you were behind on your support obligation and that they were going to intercept your taxes. Past-due child support would not have been addressed in a order to stop your current child support obligation.
If you were in fact current and this truly resulted in an overpayment of support to your child's mother, then you would need to file a petition with the court to address the overpayment. The court could enter a judgment in your favor and then the mother would have to pay it all back at once or the amount would bear statutory interest at the rate of 9%.
You will want to obtain copies of your child support payment history from the SDU and obtain a copy of the support order which imposed the obligation and a copy of the support order terminating your obligation to bring with you to any appointment with an attorney. Any experienced family law attorney could routinely handle this for you.
This response is being provided for information purposes only and does not constitute an attorney client relationship. Furthermore, I am only licensed to practice law in the State of Illinois. While there are oftentimes similarities between States' laws, there can also be large differences. You should not rely on this response as legal advice and are highly encouraged to speak to an attorney licensed in your State for an accurate legal answer.