Help? Hidden twins??!

Ariel
So about a week ago I went to my first appointment for the BFP I got back on Sept 9th. I was not keeping good track of my period last month but I was measuring at 6 weeks at the ultra sound. 
Here is where I am worried, the ultrasound gestational sack measured me at 6 week (last Tuesday) but all they could see was the gestational sack and the yolk sack, but no baby...
The doc seemed worried that it would not be a viable pregnancy so she mad me do STAT blood work and said that my Progesterone should be at 20. Well, my leaves were at 32...
Contradicting evidence right? I'm worried This may not be a viable pregnancy. BUT! Then I heard through the grape vine that there's a thing called "Hidden twins" where they don't show right away on the first ultrasound because twins are just naturally smaller but my levels would be very high!
Has anybody else heard of this? Or am I just reaching out of desperation? 
TWINS RUN IN MY FAMILY!!