What Causes Menstrual Delay?

hi , my period from february to aptil 2011 always on 4th of the month now ists already 14 of may, why im i delayed, i have an active sex life but i test this morning its negative pt.
And last april i remember after a week i finish my period i felt pain on my pelvic area then suddenlt i saw this beige color on my underwear for how many days only fee drops only