Doctors made a mistake or possible miscarriage??

The doctors usually calculate pregnancy’s from the start date of your last period.. mine was November 13th.. which means I should be 8 weeks today . And the baby should be visible with a heart beat.. But my sac empty and measuring to be 5 weeks .. I ovulated on the 21st through the 27th of November and i had sex on the 28th . Soo I got pregnant somewhere between that night and the beginning of December.. soo counting from then I’m five weeks .. so my ultrasound should be accurate right??