I'm a little confused. It sounds like you're about 6w3d or so. Your HCG is spot on with any tables I see for that timing. I would tend to agree with the MD that they should be able to see more in the sac at this point in time.
Could they see a yolk sac? Did they tell you how big the sac was measuring?
I don't want to scare you, but with my last miscarriage, I had a very similar experience(with levels comparable to yours) and it turned out to be a blighted ovum. I hope that's not the case for you, but nothing more than time will tell. I hope your scan comes back positive. I wouldn't put too much stock in the HCG at this point- if you are already at 25000, it's not likely to tell you too much (unless it goes down)
Best of luck!
With my last pregnancy I had an ultrasound that measured me at 5 wks 3 days...but we only saws the sac...no yolk no fetal pole...4 days later I had bloodwork done..my HCG was 61,219. Another ultrasound 3 weeks later showed me at 11 wks 1 day and everything was fine. My son is now 19 mos old. So you can't ALWAYS go on HCG levels. I would defiantely follow through with the repeat bloodwork and repeat scan. There's still a very good chance everything ok.