What Causes Delay In Periods While On Birth Control Pills?

ok, my girlfriend is on the pill, im pretty sure i can even tell you which one...

so weve been having unprotected sex... and if we cn move on past the "thats very dangerous, youre un responsible, youre stupid" part i would be most appreciative... well she started in november, and this last sunday she missed her period, she said she was sure she was supposed to have it. and then today she didnt get it either.... were both so scared we got her pregnant... im on my knees here... are we screwed?????