No, most likely, your girlfriend is NOT pregnant. The pill is more that 90% effective and since her period ended the night before, she was probably not ovulating (This isn't meant to offend you or your intelligence, but that means producing eggs. Just so we're clear, haha!)
Just for future reference, condoms are almost impossible to break if you use them properly. From what I understand it's usually that teenagers don't leave enough space at the tip of the penis to collect the sperm, so when you ejaculate, it breaks the condom.
I feel for you buddy. If she is pregnant, or if anything like this ever happens, don't give up on her. Please. I went though something like this when I was a foolish teen.
Good luck, I'm wishing you the best.