How far along am I

Stephanie
My LMP was 4/5/17. I went to the doctor yesterday for something else and she calculated it at 3 weeks. Don't the go from there first day of your last period?? I thought I would be 4 weeks. This app says I'm 2 weeks 6 days but it's saying my lmp was 4/14 so it's a week off and I don't know how to fix it. I just want to know where I'm at!