First U/S.. Freaking out

Lindsey
So I went to my first appointment today so excited! I knew it was early, but it was the only day that worked. Everything was going and looking great, then the tech said I was measuring much earlier. By my last menstrual cycle I am considered 7w, but I tried to explain I have longer periods and ovulate later which would put me at 6W and of course they refuse to listen
. My US shows 5w6d which didn't bother me much until she said the fetal hr was slow at 70bpm. I was told the heart could have just started beating and everything is fine or I'm going to miscarry?! I had my blood taken for hcg levels and will go back Monday to make sure they're doubling. I'm trying not to stress as I've heard of this happening to many women who go on to have full term pregnancies. Would love any encouraging thoughts or similar positive experiences!