Please help... No Sac at 6weeks, Hcg slowly rising

I need to know if others have experienced similar issues and had successful pregnancies.

11/01 - BFP

11/03 to 11/08 - Brown spotting

11/16 - Ultrasound No Sac found, Uterus measured 6wks

I've been doing Hcg testing, and my levels have slowed and I'm concerned. Has anyone experienced something similar?