Gestational Sac at 4wks6d

My doctor saw the gestational sac yesterday, which I thought was great.. however his notes say that it is “tight” as it’s only measuring 2mm. Anybody have an experience with this? My HCG was 1403 yesterday so I’m thinking maybe it’s just a bit behind since my HCG isn’t quite to 1500 yet. My RE (different doctor than yesterday) said not to worry because it was good to see anything at all, but naturally I am anxious. I’m sure on ovulation dates since I did the trigger shot, so at the most I could be 1 day off.