What Causes Irregular Menstruation?

i am 18 years old.i have irregular mens.i am getting it only if i take meprate tablet.last it came on oct 17,2014.now am taking some herbal tonic for this problem for the past 1 month,still no mens.3 years bac k i was checked for pcos,that time i took benforce tablet and it became normal,now i am not getting at all...pls give me a permanent solution for my irregular periods....plss