Empty Gestational Sac

Mika
I went in today for a vaginal U/S and they couldn't find anything in my sac. The doctor said that either they were off by 2.5 weeks or the sac is empty or may not be growing as it should. So I went in thinking that I'm about to be 8w only to find out that I may only be 5 or so or not carrying at all. Has this happened to anyone else? Help.