3rd u/s only saw yolk sac

I had my 3rd u/s on monday and only saw yolk sac, no embryo. I measured at 6.5 weeks. My first u/s was 4 weeks ago and they only saw a tiny dot, the u/s tech said "that is definitely the start of something". At my 2nd u/s, which was 2 weeks ago, the said the sac was empty. Now at my 3rd u/s they found only a yolk sac. The dr called and said "it doesn't look good" but that we could wait another 2 weeks and do another u/s. I am trying to be very positive and keep hope that I'm just too early to see a HB. I haven't had any cramping or bleeding, feel very pregnant. Am i being too hopeful?? Has anyone been through this and had a positive outcome?? I'm getting very nervous!!