What Causes Missed Periods With Negative Pregnancy Test?

i was on the pill for about 2 years i stopped taking the pill in january because i husband and i are trying to get pregnant i have had normal periods from january to july and on july 16th was the first day of my last period its been 2 months and no period and i have taken a pregnancy and its negative whats wrong with me??? how can i find out when im ovulating so i can get pregnant??