Menstrual flow less,please help!

Im 19 years old ,i used to have normal flow during my periods but it was not regular though..i used to use 1 pad per day..only on second day it used to full n on other days half or three fourth of pad used to get covered..but now i m having less flow..on the second day only 1/4th of pad is covered..n color is browninsh black too..
wat to do?

am i suffering from some disease?
or is it bcoz of stress coz i had exams!!
and im a virgin