What Causes Irregularities In Menstruation?

Hi, Ive been said i have endometriosis and has been prescribed with femilon for 3 months. my issue is, according to my gynaecologist, i take the pills for the 21 days and i should get my periods/withdrawal bleeding on the 2-3 day on the 7 day period. but i haven't got my periods yet. do i continue with the 2nd cycle on the 8th day or do i stay till i get my periods? I am not yet married and am 25 years. Please advice since my 8th day is on the 31/05/2011. Thanks.