Slow fetal heart rate 6w5d

Allie • Married & im a step mom.. Expecting baby Brooks April 1, 2017.❤️
Has anyone had a slow fetal heart beat and had a normal pregnancy? I went to the er for some cramping and they did a ultrasound. The baby was 6 weeks 5 days and had a heart rate of 91... From what I been reading it should be 120 ish by now. I'm kinda freaking out. Any advice would be nice.