Baby heartbeat

I went to ob for first ultrasound and visit today. LMP started 10/27 so it puts me around 6 1/2 wks. I don't know if I ovulated early or late though. The sonographer today said baby looked good and let us hear the heartbeat through vaginal ultrasound. Then hubby and I waited to see the doctor. She also said everything looked good but they want me to come back next week because baby's heartbeat was at 85 and they would like to see it around 100 next week. She repeatedly said not to worry as it could be normal. And all my family has said not to worry either, that 85 could be normal for 6 wks. I'm not freaking out yet but obviously concerned, has this happened to anyone else??!!