What Causes A Delayed Menstrual Period With Negative Pregnancy Test?

Hello, I am 25. I weigh around 70kgs, height would be 5'4". I was on Pills for about a year as had polycysts in my ovaries. I was off pill last to last month. Last month i got my periods 3-4 days late. This month my period is 8 days due. Had HPT done today at home..was negative. I was wondering whether i am pregnant. Any thoughts ?? Thanks