Heart rate 99bpm at six weeks one day

Hey everyone! I had to have an ultrasound today and it was found I am not seven weeks but just six weeks. The babies heart rate was 99 and the doctor said she likes to see it 100 or greater....has anyone else had a heart rate that was a little low? I am trying not to be nervous but of course I am! She said she could have caught the heart rate right as it was climbing since it's so early! Just praying when we go back in two weeks the rate is normal.