What Causes Abnormal Vaginal Bleeding After Taking I-pill?

i had sex with my boyfriend 2 year ago and i took i pill but never faced any problem..after a long time gap we had sex 2 months ago and i took i pill 2 times in a month..after few days my bleeding has been started and stopped within a few hours..few days later it happened once again and stopped..my period is normal.after 1 month we had sex again and i saw blood during that time..after that day we had sex again and i took i pill..1 hrs later my bleeding has been started. and few hours later it has been stopped.. i dint took any medicine.suffered pain also.is it any serious problem? what should i do?