I am misscarriaging now :( 1st pregnancy

I am misscarriaging right now, Inwent to dr for US and I am 5 weeks with nothing shown in the uterus + my hcg now is 820 which was 1 week ago 320 so it is not doubling so they told me i am misscarriaging

The question now is that i have blood when i wipe only and yesterday I had sex so i found blood on him but when I wipe no blood so i want to know how is misscarriaging so early ? How did you get prego after that ? Any stories ? How did you know your ovulation after that ?