What Causes Nausea And Abdominal Pain After Taking I-pill?

i got my periods on 22nd august and i was expecting my periods to come on 17 or 18th of sept.. around 11 or 12 of sept i took i pill and after 2 days i started feeling vomiting and and nausea..but my periods didnt came on 18 ,it came on 20 and i had pain in my lower abdomen that as usual and but i was still feeling nausea and bleeding remained only for 3 days( usually it remains for 5 days) and still i m feeling nausea and a very little pain in my lower abdomen..so wht is the problem....