6-7 week ultrasound no baby

Im pregnant with baby number 3 i went and had a dating ultrasound today as my periods are not regular so we cant use that to know how far i am well there was only a sac no baby. The ultrasound tech said she thinks i am about 6 weeks. Id estimated about7. When i was pregnant with my son i seen him and heard his heart beat at 6 weeks. The tech said its probably to early and my doctor will send me back in 2weeks for another ultrasound. Has anyone else had this. I cant help but be worried.