I second the others' comments. You might want to give the Relationship Community a try as well. Best of luck to you.
You know for sure that it can't be your child? Then you have some deciding to do.
First of all, you have got to consider how much you love/feel about this girl and if you want to make the relationship work. She lied to you....for months, is that something you can forgive her for and move on? Also, how would you feel being with and raising a baby that is not yours? Do you know who the father of the baby is?