I just found out my GF of 2years is pregnant; and married?

I'm losing my mind over here. She is 19weeks along and she just decides to tell me she's married in the worst way while we were arguing. It's like now she doesn't even love me and she swears the baby is not mine but I know it's at least 50/50 that little girl is mine. This is crazy, what can I do? I want her to stay with me but now she's pregnant she hates me. She said I ruined her life and she's going back to her husband. I have no idea who this guy is! If she leaves I don't know where to find her, I am mortified. How can I stop her from leaving and taking my baby with her?