Upset, but trying to stay calm.

Had my ultrasound done today and she measured me at 6w3d instead of 7w5d like my app says. Doctor said she doesn't want to speak too soon and say I miscarried until we get my bloodwork back. She said it's good that the sac is growing. I'm really worried. All we saw was just a sac. No baby, nothing. I'm really, really said because I got my hopes up to see our baby. I'm praying and hoping for the best. Doctor said maybe I'm just a late bloomer. Has this happened to anyone? I'm trying not to think about miscarrying.