Heart rate dropped


I wear a smart watch and since I fount out I'm pregnant my heartrate has been tracking as much higher than usual usually between 110 and 120 max consistently (I read this is normal but not sure, I'm 6 weeks). Today it dropped back down to about 90. Should I be concerned? I have an ultrasound on Friday but just seeing if anyone else tracks their heart rate or would know if this could mean something is wrong.