Did they tell you how big your sac is measuring? I had an ultrasound done 2 weeks ago when, if you go by my LMP, I should have been 5w 3d and all they saw was a gestational sac that measure 4w 4d which is more accurate if you go by the length of my cycles and when I ovulated (I have 31-32 day cycles). Today I went back for a follow up ultrasound and there is now a baby and a yolk sac (which wasn't there either before). I was even able to hear the heartbeat. I think that they should give you another week or two and bring you back for another ultrasound. You may have ovulated later than what you thought. Also my HCG levels were 2005 two days before the first ultrasound so they should have been above 4000 when I got the ultrasound done. Are your numbers doubling every 48 hours? If your numbers are going up then I would def. wait until you get another ultrasound. Did they schedule another one? Well I hope everything turns out well for you. Good luck.
Well the only way to know is to wait and have another ultrasound done and have your levels rechecked to see if they are going up... I am sorry that you are going through this and I hope that everything turns out okay for you =)