Second ultrasound needed

Two weeks ago, I had my first ultrasound at 10 weeks. We heard and saw the heartbeat and the baby was moving around. Today, the doctor called and wanted to schedule another ultrasound for this week as soon as possible, because the techs saw something that "didn't look right" and he wanted to do it himself. I go back Wednesday morning, but I'm so scared that something is wrong. Has anyone had a similar experience?