Ex Taking Money From My Retirement Account

I received a few emails from my retirement account about a pay request. I contacted them and found out they received a court order to withdraw $1000 in unpaid child support. Also a $600 processing fee. My son turned 18 a out a year ago. I paid exact child support on time for 12 years. I stopped paying a month after his school year ended. There is no way I should owe anything! Also, I was NEVER served with any paperwork, nor have I received ANYTHING in the mail. How can they go straight to my retirement account? Why not withhold pay or tax return? Someone please explain how this is even possible.