Positive test but no sac?

Hi Mummies! I’m a first time mom and I’m super anxious! I missed my period and took a home kit test which was positive and I calculated we are 5 weeks pregnant! We went for an ultrasound today but no sac was detected. Doctor mentioned that it could be that it’s too early or it could be an ectopic pregnancy! This has made me scared! Has anyone had similar experiences? When did your sac appear? #firsttimemom #advicepls #firstbaby