That is odd.
Here in this state Maine it is noted as a child support payment and does go as such on records, when given to the custodial parent for past due child support owed as such a payment to child support and should be a credit towards the child support owed.
You need to check with your State Child Support as you did not mention the State you are in. But where it is federal it should be with all states, not just one.
But you need to check with the Child Support Enforcement of your state as to why it was not put down as a payment for past due child support and why you were not credited for the amount as of yet. You do have the right to call and ask as the non custodial parent and it is your money that was taken away. So you do have the right to know why.