How long does OV cramps last?

I am 23 yr female, regular 26/27/28 day cycle but never miss AF. I have been TTC for almost 2 yrs after a MC, Last AF was 8/19/2010 28 days, my OV cycle was suppose to be 30-03 i have been cramping since the 31st i BD on the 31st, 2 OV day and i will try today again, but i wanted to know why do i cramp for so many days? Thanks Girls