What Causes Delayed Periods After Taking Postinor 2?

my wife took postinor 2 an hour after we had had unprotected sex and took the second pill exatly 12 hours afterwards. since then she has not had her period for almost 2 months now eventhough she has done series of pregnancy tests that proved negative. she experiences cramps once in a while and yet doesn t see her menses. what could possibly be the problem?