egg sac, no yolk/pole - doubling hcg in 24 hrs, 28.5 progesterone

On June 10 (Wednesday), I had an HCG quantative test performed.  The results were 6,465.  On June 11, began having minor brown discharge, so went to OB/GYN.  Ultrasound performed showing egg sac, no yolk or fetal pole.  The doctor was concerned, being that my first day of LMP was around April 20.  He sent me for more bloodwork, expecting to see HCG decreasing and low progesterone.  

Instead, my HCG level had doubled in an exact 24 hour period.  The new level was 12,179.  Also, my progesterone was 28.5.  The Dr said 25 was good, so he was actaully suprised by my results.  He's scheduled another U/S for Tuesday, June 17.  Until then, I'm going crazy.....

I've read with a blighted ovum you can have an increased HCG for a while due to the placenta not yet knowing the pregnancy is not viable.  However, is it normal for HCG to double this quickly in a blighted ovum case?  Is it typical to have good progesterone levels in a blighted ovum case?

Could it just be too early to see the baby on U/S?  It's a 4D machine, but only 2D screening was used (transvaginally).  I'm not 100% sure about the April 20 being first day of LMP, but I know for sure that I did not have a period during May at all....so I'm at least 6 weeks.  

Thanks for any information you can give me.