blighted ovum

Went to my what I thought was my 8 week ultrasound but the sac was empty. Of course they said there is always the chance that I'm not as far along but you think something would have shown . Has anyone had an empty sac and then something grows? I hate doctors walking on eggshells and not giving me a straight answer. I had blood work done and will go back in a week for another ultrasound. I haven't had any bleeding or anything. When will my body get rid of the sac on its own if nothing is growing? This has really got me in a funk as my first pregnancy was perfect and now I feel like I failed.