supportkids,inc Austin, Texas continue to harass me for money after I paid off child support in January. Austin Texas

I paid off child support in January 2017. It was garnished from my wages at EcoLab. Ecolab notified me that the money was being returned due to pay off in January,2017. EcoLab informed me that they were not having much success communicating with this company when I asked for documentation. Today, November,13,2017 I received a letter from suppportkids stating that I owe them money. I have received no phone messages, no emails, no text messages, and this is the first that I have heard from them. Ther is no invoice, no statement, no record of any kind. There is no balance listed. I called the phone number on the letter and left a message, with no response.I do not know if this is legite or a scam. I assume that this is the company that was collecting because I was never given any information.