Gestational sac no baby?

So I had my first prenatal appointment yesterday and it did not go as planned - I believe I am somewhere around 5 1/2 weeks along. They went over everything with me, what I could and couldn’t eat, got me signed up for genetic testing, and did the transvaginal ultrasound. There was a small gestational sac but no baby. My dr said I miscarried, have an ectopic pregnancy, have a blighted ovum, or am earlier that I thought. I went and had bloodwork done and my HCG levels are around 21,000. I just don’t understand? I’m having all of the symptoms, positive pregnancy tests that are getting stronger everyday, but she made it sound like there definitely wasn’t a baby in there and crumpled up the ultrasound photos and said “I guess you can take the pamphlets home still”. I’m just so confused because it’s only a little over 5 weeks and from what I read just seeing the sac at this stage is normal? I was crying in bed all day yesterday - I went back today for another blood sample and will see if my levels rose tomorrow. I just don’t know. Has anyone else had this happen and everything was okay? I have had Only light cramping and NO spotting at all, like at all. I just don’t know what to believe.

*****UPDATE:

You were all right! It was just a little early :)******