Please help. Possible chemical?

Got my 1st BFP dec 02. After being diagnosed with pcos and trying for over a year, i was over the moon ecstatic with the news.

Based on my last period i should be about 8 weeks.

Had my 1st ob appt today and they started with an ultrasound. When the doc said there was no sac my heart dropped and i felt like my world was crashing down. So she tested my urine and said it was positive and maybe my dates were off.

But i havent had a period since october. She sent me to have my hcg tested, its 728.

I go back saturday to have it tested again and see if it goes up or down.

Has anyone had any experiences that they can share to help me understand what could be happening?

***Update***

So i had my 2nd hcg blood draw today...it hasnt been exactly 48 hours, more like 40 hours...but it did increase to 840.

Im happy for the increase, but its not double like most articles say it should do every 48 hours.

Any input? Thanks in advance