I am 49 years old and just recieved a court order to pay child support for a child born in 1986.?

I am not the father and I asked the state of Illinois how can this be. According to the state of Illinois paternity was extablished sep 26, 1986. First I haven't resided in the state since Jan 1985 and can prove of my where abouts in sep 1986. I recieved a response that paternity was established in 1986 and if I want to prove I am not the father then I must go through the porper legal channels. My question how can the state of Illinois establish paternity when I never submitted to a paternity test or signed a birth certificate. Next the parent has passed and I have no knowledge of the whereabouts of the child in question..