Early scan

So I went to er because I was having bad cramping I was scared b/c previous m/c. I thought maybe I was 5 to 6 weeks from lmp. They seen nothing on the scan. No sac nothing. Hcg was 335. The lady doing scan was like well your not 5 or 6 weeks there is nothing here to indicate any pregnacy. But then Dr comes in and was like your 3 weeks pregnat.. Does any of this make since to anybody? Why did they see nothing on scan? Any words of advice would help. Thanks.