6 weeks can't see heartbeat

I went to the doctor today thinking I was 8 weeks, and baby is measuring 6 weeks and we couldn't see a heartbeat. I am going back next week. She said she was concerned enough to hold off on doing blood work, but also not concerned since my period was messed up I may have ovulated 2 weeks later. I took and got a positive pregnancy test January 19th which is 2 days after I was supposed to start my period. In the sonogram the baby looks much smaller than in my now 2 year old daughter's did at 6 weeks. Can you even make the comparison? Anyone else in this position? I know 50% of time you can't see a heartbeat at 6 weeks, but just feeling anxiety.