Am having a tender breast nd it increased in size nd also headache nd dizziness can it be the cause of postinor2?

Hard to guess. Of course, one should do a home urine pregnancy test 3 weeks after sex (presuming sex was the reason for taking Postinor 2). If not pregnant, then one can ask her primary care doctor about the symptoms. As long as one's menstrual cycle has returned to normal after 1 (or maybe 2) cycles, it is unlikely that her bodily symptoms are due to past emergency contraceptive pills.