missed period but possibly still pregnant because of ovulated later?? help please

I'm 14 days late for my period, I've suspected sometimes I ovulate twice in a cycle, my partner and I had sex 3 days before period was due and around ovulation... my tests have been negative, my ultrasound shown a thickened endometrium and a bulky uterus, I have had other symptoms.. I know doctors go from LMP but could it be possible to have conceived later on and its not showing up in hcg levels because its so early? I am always regular on the dot with my period, only 28 and fairly healthy?