Can I tell you a secret? I may have gotten a one-night stand pregnant, but I'm already in a great relationship with someone I love.
Are you joking because why would you have a one-night stand when you're already in a relationship?
I don't know, I wasn't really thinking about the possible consequences at the time.
Have you come clean to your girlfriend? Cheating on her is wrong, and lying to her is no way to build a healthy relationship, either. Instead, this kind of behavior destroys relationships.
I haven't told her yet. I don't want her to leave me.
Only the truth can set you free, because there is no point dragging this lie on because a baby is involved. Just tell her how you messed up and beg for forgiveness. Maybe by some miracle, she will not leave you.