Should be 8-10 weeks but ultrasound shows 4-5 weeks

Maligha

I had my sex on May 15th bled lightly may 20th -24th. On June 11th -13th I spotted on and off. I got my first positive home test July 4th. On July 15th I got an ultrasound and the nurse said I was only 4-5 weeks when I should be farther than that. This is my 2nd child and I didn’t have this problem with my first. Anyone have any idea what’s going on??