Confussed, 7 week ultrasound

Laura
So I had a ultrasound last week and the heart rate was low so my doctor advised me to get another one.
Last week baby was 
0.35-0.37 cm
Heart beat was 
91.46-92.62
This week baby is 
0.70-0.79 cm
Heart beat 
133bpm 
Not sure why but it says 6 weeks 4 days it's measuring but that's what it measured last week has this happened to anyone I'm confussed ??