My Girlfriend Is On Birth Control And She Missed Her

My girlfriend is on birth control and she missed her period . She has missed her period before for 3 months. she stopped taking her birth control pills for 4 days because she isn't suppose to take them on those days. She took her last pill Tuesday and then we had unprotected sex Sunday and she took her birth control that night cause she started a new pack. I'm scared should I give her plan b ?