What Causes Irregular Menstruation After Taking I-pill?

Hello Doctor i had an unprotected sex with my boyfriend and did took ipill withing 4 hours. My periods were due after 3 weeks when i took ipill but i had it in next week. i had seen on internet that it is a very common side effect of Ipill. hence i wasn t very worried, but now my second period has got delayed by a week and still it hasn t started. kindly help!