Has one been told that they had a blighted ovum and be missed diagnosed? even if not miss diagnosed please hel?

today i went to get a sonogram to see how far along i was, the u/s tech told me that i had a blighted ovum i was so shocked and told me that i was gonna have a miscarraige she said that it looked like i was six weeks amd i was supposed to be around nine. the reason i say about nine weeks is because i guessed the day of LMP but truth is i dont remember, what im trying to say is what if they were wrong and i am just six weeks shuld i go to another hospital, i have anther doc appt on monday the 28 if there is still no miscarriage what should i do?