Large Yolk Sac

I just found out I'm pregnant again and went to my first dr appointment. I'm 6w3d and dr could not find a heartbeat which she said they usually see at this point. She also pointed out that the yolk sac was very large. She said it was not proportionate to the baby (it was about half the size of the baby) or to my gestational sac. Because of this she ordered additional tests and wanted my hcg levels checked. I will not find that out until tomorrow and I go back for another US next week to check on the baby. But she said she was very concerned by the sac size and the lack of heartbeat. Anyone else experience anything like this?