This is a tough situation that I went through about two years ago. First and foremost, any good attorney will advise you to start taking notes. When I say notes, I'm talking times, dates, and specific details of stuff that has happened that involved you, her, and your child. Secondly, get a DNA test. If you question the legitimacy of your child, you need to get a DNA test. That's hard to do, because it can turn out how you don't want it, but it's for the best in the long run.
Like I said, detailed notes are a dream come true to any judge or attorney overseeing a case. They need to be 'unbiased', but that's hard for someone in your position. They need to be notes like, "[insert name here] came to pickup [insert name here] and called me a [insert bad word here] in front of our child, in the process of picking them up." They can't be "The dumb bitch came to pickup my kid and the bitch called me a [insert bad word here] while she was here."
You need to keep your cool, and use the legal system. When all is said and done, you may get partial custody at best, unless you can prove she is an unfit mother, which with good notes, you can do. Right now, you're shooting for partial custody with visitation in exchange for child support.
If you need anything else, let me know. I've been through all this and it's tough.