Early pregnancy test!

I found out I was pregnant at what I thought was 3 weeks and 5 days. I was thinking I was 9 weeks 2 days at my first appointment. Turns out, I was off and only 8 weeks and 5 days. That would mean I was 3 weeks and 1 day. I know that isn't a lot of, but would a test even be positive at that point? Anyone been off on their time at their first OB appt? 9 weeks 2 days lines up with my period. So I'm confused and the OB was as well. Maybe my baby is just small, but the OB didn't even say that, she just said I'm gonna get an internal ultrasound to confirm. Again, would a test be positive so early? Anyone else's dates been off from their last period?