Only gestational sac?

I was 5 weeks 4 days and had to go to the ER for cramping so bad. Well 5 hours later I got an ultrasound. I couldn't see the screen though. They said there was only a gestational sac. In my last pregnancy I got an ultrasound at 5 weeks 2 days and we seen a tiny baby. Has anyone ever had this happen?
UPDATE: went to my OB. Definitely not empty. She seen a yolk sack but no baby. I swear I seen a baby though. Go back next Friday. 😊