What Causes Clotting With Heavy Bleeding During Menstruation?

I am 22 years old and I took postinor 2 for the first time on the 20th of march.My last period ended on the 19th of March.I am now having another period which started on the 2nd of April but before my first period,my breats became tender and remained tender until this second period started.Also,my period is accompanied with pains and a lot of clotting with heavy blood fow.Is my period as a result of the Postinor or something else?