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Stress plays important role in periods. She might be freaking out with the fear of getting pregnant, with exams on the head causing a delay in her periods. Please consider doing a pregnancy test, most likely, it will be negative, and that will help her relax. Periods will gradually occur. No need to worry. Yes, there are medicines for getting periods, but they might worsen her condition by causing mood swings, as she already has taken an emergency contraceptive.
Secondly, emergency contraceptives should be taken within 72 hours of unprotected intercourse, the earlier, the better effect. She has taken it quite early even before intercourse, so chances of pregnancy are nil.