Cant see baby in vaginal ultrasound

Daisy

Today I went into my OB/GYN to get an ultrasound and according to my last period I am seven weeks and two days but when I went in for the ultrasound they couldn’t see anything. Just a gestational sac. They said i might be earlier than they thought and they took blood test for hormones and testosterone and I go back in 48 hours to see if they are normal. Has this happened to anyone else. I am worried because about 3 to 4 months ago I had a miscarriage.