Please advice.. desperate

I went for a viability scan, I should have been 7weeks going by my last period, however the scan showed I was measuring only 4 weeks. There was a 2.5mm sac but nothing to be detected in the sac. I’ve been told there is 3 directs this could go, either i ovulated late and it’s just too early, its not a viable pregnancy that will end naturally or ectopic as they couldn’t be 100% sure the sac was in the right place.

I’ve been advised to prepare myself for any of these options until my rescan in 3 weeks time. I’m just after some real experiences. Has anyone been scanned at this point and only had an empty sac that was a viable pregnancy or did it end in one of the other ways?

Updated-

First obvious positive was 17th first faint was the week before.

Thank you in advance.