What Causes Irregularities In Menstruation?

hi i am female and 25yrs old.i got married 5 months before and i am using birth control pill since my 1st month of marriage,from that time my menstural cycle has been reduced i was getting 5 days of menstural cycle before and now i had 3 days of cycle??should i need to consult a doctor?? is this a serious problem!!!