Moved Due Date, Low Heart Rate

Went for my confirmation appointment and everything looked good. Dr scheduled an ultrasound to make sure due date was on track and for viability. According to LMP I was 8 weeks, ultrasound showed 6 weeks, 3 days. I saw the heartbeat, but nurse practitioner said it was lower then she would like to see at 102, and scheduled another ultrasound for the next week. She also moved my due date from May 25 to June 5.  This is my second pregnancy and never had any kind of problems with my daughter so I've been very worried. Trying to stay positive and praying! Just curious if anyone else had anything similar happen to them.