Heart rate

Hi everyone, 
I had an ultrasound today . A doctor (fellow) told me that my baby was measuring 8 weeks and 4 days (which is right where I am supposed to be ) and the heart rate was 115. I asked "isn't that low?" And she replied "the baby's heart can flucate throughout the day and it could be resting. Everything looks great!" I had some back pain and cramping prior so that is why I was seen. 
I haven't seen heart rates that low since my 6 week 0 days appointment. 
I am worried. Anyone seen anything like this?