First ultrasound 6w3d heart beat slightly low?

Ultra sound at 6w3d. She said I was messing smaller but my ovulation date could have been a little off but I have to have a rpt ultrasound because my hb was less then 120?? My hb was 109and she was guessing I was about 5.5 weeks? Everywhere I look online that is normal for that far along but the practice likes hb to be about 120. Scared about going back for a rpt and it not being high enough now, but she made it sound like it would be fine.