What Could Cause Delay In Periods?

hi.doctor,myself kalpana,female,age 25 yrs,m still single but going to get married in dis december,actually in nov 15 2009 i had sexual interaction with my fiance n after dat i got my date on 29th nov 2009 very regularly but after that i dint get my date for next dec n january month thn again got it in feb n mar after that till now i dont get any date from past 4 months..i have cosulted to doctor dis month only as i get busy in my work all the time n can say a bit careless regarding this bt now my doctor suggested me to hav PCOD test n given me MEPRATE tablet its been 3 days i have stop taken this tablet bt still i dint get my date..pls suggest me what kind of life style i should adopt as i need to take lot of stress regarding my work..i am worried about infertility also as one of doctor have also suggested me that i have a bit low fertility rate.i very much worried pls do needful n guide me to the right.
thamks in advance