Should i have been given an order on back child support

I have an order in 2015 for support , which i pay i am up to date with my payments, this month i recived a letter i owe 12k in back child support including almost 4k in interest. I had no idea of this order, and if i had known i would have payed month to month or it should have been deducted from my paychecks. Now im scared i will go to jail because i cannot pay that amount upfront and im paying my normal payments. How is this legal to tack on so much interest on something that never existed, i didnt even have my rights for a year of that time, and another year i was denied visitation. They knew where i lived so why wouldnt they have sent me anything so i wouldnt have been so behind.