First appointment today....concerned.

I had my first appointment today. According to my last period, I should be about 10 weeks. My OB was unable to for certain find anything on ultrasound; she said she saw flutters and a sac but we were unable to see a fetus or hear a heartbeat. She’s having me come back in two weeks to do another ultrasound, and is thinking I’m simply not as far as we thought, but I can’t help but be concerned. Has anyone else had this or have any advice??