I had an unptotected sex by novermber i used postinor 2 immediately since den i have had an irregular period sometimes 26days cycle sometime 36 days cycle i do t know what could be the reason?

"Regular periods" are generally defined as between 24 and 36 days apart. Variations in this can be accounted for by delays in ovulation, which may be related to "stress" (work, school, emotional, etc.) which will also delay the onset of menstruation. Oral birth control will stop ovarian production of hormones, making the cycles, although "artificial", very precise.