measuring behind

I am pregnant with my first baby through letrozole and trigger shot.

On the 28th of january I started bleeding a little and when for u/s. I thought with lmp that I was 6w6d at that time but accourding to the u/s I was only 5w4d. They could see a fetal pole and a heart flicker but I was measuring 9 days behind. I am worried because I took the hcg trigger shot on cd 16 and most likely ovulated on cd 17 or 18. I should have been around 6w6d but I am measuring behind and it worries me. Has anyone else experienced this and had a succsessful