6 weeks 3 days and measuring behind

Hi all! I went to the doctor for my first ultrasound today, and I'm still feeling uneasy. I'm hoping to connect with someone who may have experienced something similar, whether it's a positive or negative outcome...
My period has been very irregular after I stopped pumping with my first daughter, who will be a year on Sunday. According to my LMP, I should be 7 weeks and 5 days along, but I only measured to 6 weeks and 3 days. I know we conceived either December 7 or 8th, because let's face it,  we're hardly doing it with a little one who still sleeps in our bed 🙃 By those calculations I should be about 7 weeks and 5 days as well.
Anyone else measure this far behind? I should also say the heartbeat was 119, but the doctor likes to see 120. I'm going back next week to see if there's progress.