Pregnany Concern

I just founded out that I am pregnant last Sunday. I started bleeding very bad and went to the ER on last Thursday. My Hcg level was 3000, they said I should be about 5weeks. However they could not find the sac on the ultrasound. So I was to come be in 48hrs to have my Hcg levels recheck on Sat May 2. After checking checking a second time. My Hcg levels were the same as they were 2 days before. So we did another ultrasound and again could not find anything. Has anyone ever experienced this? I'm not sure what to think. I have to go back on Monday to have my levels checked again.