What Causes Irregularities In Menstruation?

i had delayed menses in last month...so i was prescribed norethisterone tablet ip 5mg (primount-n) twice a day to prepone my menses,and i got my menses in a week.this month my menses have further been delayed by almost a month...i did pregnancy test twice at home(HCG) it was negative.could u please tell should i consume another dose of the same medicine to get my menses?
also...before last month..i had regular menstrual cycle
please help