How long ago did she have her period? Something a bit more reliable is to figure out when she was ovulating, while she can get pregnant when she isn't ovulating, it's pretty rare. You can figure out when she was ovulating (about) using http://www.babycenter.com/calculators/ovulation/ <- that. Hopefully she bleeds and none of this is a problem though!

Don't worry hon, That's happened to me two or three times before (and torn condoms too), be sure that both of you don't stress too much until you know facts not assumptions and try and get her on birth control or have a plan/go to planned parenthood and talk to someone there. It sucks but there's a shitload of responsibility attached to having a nice bang