15 Year Old Potential Father And Not a Clue What to Do Help!

Early today at around 7pm or so, my girlfriend of about 2 months called me and told me that we needed to talk. I figured oh s**t I hope this doesn't have to deal with when we had sex a couple of weeks before. That was the only thing on my mind. That it could be a million other things but for some reason I knew it was going to be that one. When she left her house for a bit she called me back and then she told me that she thinks that she's pregnant. At that exact second my heart dropped in the to pits of my stomach. Knowing that this night was indeed going to be a long one. We talked it over and over and over. I immediatly asked her questions to see if she was pregnant. I asked her how long has it been since she should have had her period. She told me eight days. That she usually gets it on the twentieth of every month. So I figure maybe a month of regularity. Then I checked and the last time that we had sex was on the twentieth... Once I made that realization, I felt dead. That I have now f***ed up the life of the person that I love the most and mine. We used a condom every time that we did do it. It didn't rip it was perfectly intact. Im wondering how the h**l could it happen then. We were safe. The condom fit me fine nothing came out the side of it onto my scr*tum or anything. I don't even think I and a huge amount of semen come out that time. That is why I am so confused why she is late. How could she be late? By eight days? Is that normal for a fifteen year old to be off a week or so? Is it normal for a 15 year old to be going through a situation like this? I never thought that this would happen to me, or that I would do this to someone at this age. To get them pregnant. I know await for her to confirm with a test. I don't know how in the world im going to get through this night. Please reply back with some answer to any of my questions. Thank you for taking the time to read this.