7wks or 5 ? Possible empty sac ??

About 2 weeks almost 3 weeks ago I went to the clinic and had a negative pregnancy test. I had several positive at home. I knew I was pregnant. I only missed my period by a week at this point. I was told I more than likely had a miscarriage and that’s y my home tests were positive. I didn’t feel right about that explanation at all. I finally had time to go to a doctor just this past week. At this point idk what’s going on. I tell the doctor and she tells me I am about 7 weeks pregnant. I get an ultra sound and there is a sac but no yolk and no embryo. The sac measures out to be about 5 weeks gestation. How far along am I ? When should they see something other than a sac ? Should I be worried ?