Empty sac

Im very confused, and Im goping somebody can give me some insight. I went in for my US thinking I was somewhere around 8weeks. Instead thwy found an empty sac measuring 6weeks. We are doing blood work, and I have a follow up US scheduled. Has anybody had this happen? My PA didnt seem too concerned, she prepared me for both posibilities. My hesitation is that I had my BFP 6 weeks ago, so I dont think the sac could be 6 weeks and still be viable (accounting for the two weeks before conception). Because if the sac is measuring 6 weeks, that would actually be 4weeks gestation? Correct? My mind is going 100 miles an hour trying to figure out if this is possible or not.