Hcg levels and ultra sound findings

So I had a miscarriage/chemical pregnancy towards the end of July. And I ended up pregnant sometime in the middle of august(so soon) my dr has been tracking my hcg. I had some cramping, no bleeding and went to urgent care. My levels are GREAT. But on the ultra sound it shows nothing. They said they should have been able to see the gestation sac Atleast with my hcg being 1856. I have another appt and ultra sound on Tuesday with my obgyn. I’m curious if anybody has experienced anything like this? Where there’s nothing showing on their ultrasound but hcg levels are doubling and going as they should. I’m so anxious.