What can I do if my ex tricked me into signing a voluntary paternity establishment for her daughter?

I went to her house a few days after she had her daughter to see my son and she told me to sign a paper for her that was in regards to my son.Then I started having to pay double the amount of child support.I was going to the child support office every week for a full year asking them what child I was paying child support for, and the whole time they were telling me it was only my son.I also made an effort to contest paternity for her daughter and a hearing was never given.So now its a year and a half and only now I was notified that I am still paying child support for my son and her daughter.