What Causes Missed Periods And Negative HPT While Having PCOS?

Hi..i am 23 yrs lady..been diagonised with pcos..i was on birth control for 2 months and then i had my normal periods on 07/20/2010,now again this month i did not have my periods..i did three pregnancy test and they came out negative...i had an yeast infection for which i was given a pill...now i am having stomach ache from last two days...i really want to know when i can be sure i m pregnant bcoz i have already missed my periods and my preg test r negative..so can u help me?