Hi I Went For An 8 Week Vaginal Ultrasound Yesterday

Hi i went for an 8 week vaginal ultrasound yesterday and there was no baby nor no yolk, they want me to come back in 2 weeks to check again. I have read others posts about misdiagnosed miscarriages, is that really possible? If so, what are my chances, I also may be one week off making me 7 weeks instead of 8.