Hi, thanks for your question. Emergency contraceptive pills are effective at preventing pregnancy if taken within 72 hours of unprotected intercourse. However, the rule is the earlier one takes it, the better, and the lower the chances of pregnancy. Your girlfriend took it early enough to prevent pregnancy, so you need not worry. However, all cases of delayed periods need a laboratory exclusion of pregnancy first. So i would advise you take her to a primary care doctor to run a pregnancy test. In the absence of pregnancy, other causes of delayed periods are evaluated. These include stress, sudden weight loss and weight gain, change in eating patterns, and ovarian diseases. An ultrasound scan would be used to detect any problems with the ovary. Once the diagnosis is found, appropriate treatment would be administered. I hope i have answered your questions. Please eview with me if you have more questions. Warm regards.