Had a ultrasound and they only seen one sac no baby

Lacey • Married:) one miscarriage and one rainbow baby! Took us 3 years of trying Jacob 5-18-16 my whole world
Has anyone had a ultrasound and had them only see the sac but it was empty?? I'm supposed to be 6 weeks and 2 days I had it yesterday they told me to go for blood work to see if my numbers have gone up which they have they went from 6854 to 22748 I go back Monday for another ultrasound they said it could be to early and are happy that my numbers are going up