Hospital is wrong?

Lindsey
The first day of my last period was April 27th I had a period app that I followed. Any way I will be 9 weeks tomrrow but I went to the hospital(er) last Monday they told me I was only 6 weeks 4 days. 
They didn't do an internal exam, it was only on the belly. So could their measurements be off?  Even the sono lady said according to my last period  it should be measuring  differently. 
This would also mean my morning sickness started at3 weeks?? 
I'm very confused