What Causes Missing Periods And Negative Pregnancy Tests?

hi,i am 26 years old.i got married 2 years before&i had2 missed abortions.my follicular study shows that my egg ruptures on 13 th day with endometreium thickness 9mm.we undergo sexualyy active on that day& the next day.now my period is missingby 2 days but urine test shows negative