Once your levels get beyond 5,000 or so then they generally stop doubling, slowing done a fair bit. Also, once you're able to see anything on u/s, that becomes a more reliable way of measuring growth. So if you go back in a week with what they saw before you should be able to see a heartbeat, and they should be able to measure how much the gestational sac has grown. Honestly, I'd push for another ultrasound rather than another blood draw. The extra blood draw just won't tell you enough at this point. HTH, and good luck!
ETA - I thought I wrote it before, but YES it is absolutely possible to still have a normal and viable pregnancy! :-)