What Causes Spotting And Missing Periods?

hi, i have my periods on 14th april2011.i hv taken 2 ipills in at the end of the month.my problem is that i bleed very little on the 24th day of the periodic menstruation cycle.after that no sign of bleeding.i was suppose to have my periods on 12th may 2011.but there is no sign of it.home pregnancy test was also negative.kindly suggest