Miscarriage??

I started bleeding last night. Called the doc and she sent me to the ER. I should be 11 weeks today based on my LMP. Vaginal ultrasound shows a 6 week 2 day gestational sac but no yolk sac or heartbeat. My HCG is at 5671 so im going to the doctor on Thursday to check if levels are rising or not. I've been in tears. ER doc is not optimistic. He's thinking a blighted ovum but mentioned we could be off on our dates. I just don't see how it would be possible to be that far off. The bleeding stopped last night, just a little spotting now. Praying for the best but expecting the worst at this point. Ughh. :(