I M Pregnant, But As Per My LMP I M

I m pregnant, but as per my LMP I m on my 6 week.Just after two days of home pregnancy test ,I had very light brown spotting. After which I was advised by the doctor to take Tab Susten twice daily for two weeks.Meanwhile my beta hcg level first came around 4000, then it got dropped to 1788, then 1500. And in ultrasound the doc was only able to see the thick and vascular endometrium but no sac . I m confused. Could it be possible that I had concieved late in the cycle.