FYI: I had a m/c about the same time and they did not see anything on the u/s and I hadn't even bleed as much as you are saying. I agree with PP the fact that they saw a sac is a good sign. It could be a vanishing twin. Many people bleed during pregnancy, especially with IVF.
Oh no!!! I saw this after I responded to your fm. I agree with BA - maybe it's a vanishing twin??? When I had my mc last year, my numbers were no where near yours and when they did the sono, there was nothing there. Hoping the sac is a good sign.
I am so sorry - how nervewracking! This happened to one of my friends and she now has a perfectly healthy 22 month old! Her bleeding was never explained. I hope it's the same outcome for you.