Could I pay child support for a child that is not mine?

I have recently been served documents regarding sections 601. 602. 602.1 and 610 of the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/1 et. seq.) Also pursuant of sections 5/21-101 of the Illinois Code of Civil Procedure(735 Ill. Comp. St. 5/21-101 et. seq.) Followed with Pursuant of Section 14 of the Illinois Parentage Act, And Pursuant to section 610(b) of the Illinois marriage and Dissolution of Marriage Act. My ex girlfriend and I had a child together; I stayed with her for nearly two and half years. Nearing the three year mark we decided to separate. We had undergone court mediation and come to a joint custody agreement. I moved out of the state shortly after approaching her about a paternity test to decide whether the child was mine or not. She denied me that request and explained to me that the child I raised for nearly two and half years was not mine, she then cut all communication with me, where I decided it was best to voluntarily relinquish my parental rights over the child. She is now serving me with this papers claiming child abandonment despite cutting all forms of communication with me, as well eliminating my visitation, rights, and name change. My concerns are that I will end up paying child support for a child that is not mine, or that I will be unable to prove that this child is or is not mine before I lose my rights. What do I need to be concerned about?