Baby measured small at first ultrasound?

Hi friends. I went in for my 8 week appointment today one day early bc I was having some light spotting. The baby was there with a heartbeat of 122 (on the low side) and measured 10 days smaller than supposed to (6 wk 3 days instead of 7 wk 6 days). The doctor wants me back in two weeks to measure again be hopefully see more growth. Now I am so worried. Has anyone had a similar experience?  I did not have this type of spotting with my first and I'm very scared. 😳😢