6+3 ultrasound

So I had an ultrasound Friday and they couldn't see a heartbeat. Just a yolk and sac, I wasn't too worried because I've read that it's hard to see that early. Then today my dr calls me and sounded super serious about the fact that there wasn't a heartbeat. And that I need blood work and follow up sonogram and all this stuff. So now I'm freaking out. I'm still getting morning sickness, some tenderness, and the craziest dreams. No cramping or bleeding. So is my doctor just being cautious or should I actually be freaking out?