Blighted Ovum? Could it be something else?

Hi there, I am writing for any hopeful stories that might be out there. I am a mom of a sweet 3 1/2 year old son. Everything with my first pregnancy was super easy including labor and delivery. I went off birthcontrol  in November and my husband and I started TTC in December. On Februray 13th I had my LMP. My cycles usually range from 28-31 days.  On March 13th I took a positive pregnancy test. I went to the doctors when I thought I would have been 6 weeks pregnant and had my OB (same doc as with my son) confirm I was pregnant (she said as she did the exam It's very, very early---which I thought was odd since I was 6 weeks but didn't think anything of it) I was scheduled for my US on April 12 which should have put me at 8 weeks. When the tech did the ultrasound she saw the gestational sac, but my doctor said I either have what is called a blighted ovum or there is a slight chance I had a longer cycle and conceived later. She said "Brace for the worst and cross your fingers for the best". I am driving myself crazy. Up until the ultrasound I was extremely tired, needing naps, breast tenderness etc.  I still feel these symptoms but am second guessing them now. I have had NO spotting, NO cramping, everything has been normal. Based on all of this  could there be any chance that the baby is just 5 weeks when I got the US or do you think that I am destined to miscarry. We were so excited for this baby and the timing couldn't have been better. My heart is broken right now. I can't sleep--I just think about this. The next two weeks are pure agony.