Okay so, my ex gf is pregnant. She doesn't know who the dad is because when we got together, she was just leaving her other boyfriend.

She was also going back and forth from my house to his to "get stuff." She called me and told me she was pregnant and I'm the dad. But she told her sister that I'm not. Now she won't answer my calls, my texts, and she won't talk to me in person. I have no idea what to do. If I'm the father, I feel like I have the right to know so I can prepare myself for when the baby is born. Also, she got me all worked up when she called me. I've already told family I'm gonna be a dad, I've bought things I need, I've grown accepted to the fact of it. But what for? When it could be someone else's kid completely. I need to know something.. please help me figure this out.