HCG vs Transvaginal US

I was having cramping today and went to the ER. They did a transvaginal US and no sac was seen.  No mass was seen within my tubes.  Only thing seen was a thick endometrium.  My periods have been irregular lately so im not sure when I ovulated.  My hcg levels went from 113-306-716-2316 today. Per charts that I've seen it correlates with 5 weeks.  I thought HCG levels over 2000 a sac would be visible.   Has anyone else experienced this?