Should I be worried or not?

My girlfriend ran out of her birth control and the day after that one day before she was supposed to get her period we had sex and the condom broke. She missed her period and hasn't got it yet it's been about 3 days after and she's having bad stomach pains she's feeling nauseous drowsy and she's having weird feeling in her stomach. She took a plan b pill the day after we had sex. Should I be worried?? How soon should I buy a pregnancy test?