1st ultrasound changed due date


So according to this app and 4 other <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">pregnancy apps</a> based off my last period (12/23/17) I am 7 weeks pregnant. I had my first positive pregnancy test 1/14/18.

I had my first ultrasound yesterday and the doctor called me this morning saying that based off that and my lab work I’m now only 5 weeks pregnant. It’s just not adding up to me with the dates. Can it really be that far off?