Too early for ultrasound

Emily • Married 20 years, mother of 4
Has anyone else gone in for their first ultrasound and the tech could not get a picture due to you being too early? I am nervous and worried but true doc said it's fine, that we just had my due date wrong. Did blood work after failed ultrasound and go back in 48 hours for more blood work to check my count. 
Confused!!!!