Your name/info listed when you post indicates you're in Florida. I don't know your story and if your ex/child(ren) are also in Florida or if you have a court order and it was venued in Florida. If so, this is what I found on the Florida child support website under frequently asked questions regarding payments...
Article No: 968 (Revised: 03/05/2003 ) Question: Will the support payments be paid directly to me? Can I send the payment directly to my spouse?
Answer: The Court Order states who receives the support payments. It is suggested to read the Order very carefully; failure to make payments may constitute contempt. If it is not instructed in your Court Order and you make direct payments to the custodial parent, you may not receive credit for payments.
If you aren't under a court order, you can present that information to receive credit for payments made between when you were separated and now.
If you are court ordered, I'd make sure that whatever child support agency is monitoring your case has your payment information. If there's a court order, someone is monitoring - depending on the state the order was venued in.