What should I do when I find out my husband has a kid with another woman?

Long story short - I have been dating the same guy since I was 14 years old. I am now 19. We have been married a little over a year. I get a phone call from some girl claiming my husband owes back child support for THEIR son. I thought I was going to die. I just cried. He says she is lying and its not his. BUT she faxed me over where he signed the birth certificate. He swears up and down he did NOT sign that. Does he have to be present during the birth to sign the BC? Or could she have forged it? I don't know what to do or who to believe. If it is his, that means he cheated on me and now I am a stepmother. What should I do? I think I have hit a breaking point. I need some help. Oh, also - she named the kid after him. His name is Anthony Patrick Ryan, the kids name is Anthony Aaron Ryan. :( Please give me as much advice as possible before I end up doing something stupid. My whole life feels like it went down the drain. I love him.