What Could Cause Menstrual Irregularity?

my wife had her last periods on 12 feb 2014 after that on 24 feb 2014 we had unprotected sex, she took ipill in 48 hours , after a week on 1 march 2014 she had spotting light bleeding for 3 days, her period was due on march 12 2014 but it did not arrive , again we took home pregnenecy test was negative , so we consulted gyneaocologist on 8 april 2014 he suggested regestrone tablet 3 times a day for 3 days she took the tablet but now till 21 april her periods did not arrive , we had beta hcg test which was negative , we did pelvis usg which was normal saying no abnormalities seen. please help what might be the problem and when will my wife be getting periods .