Need Advice 😔

I went to a clinic and got an ultrasound two weeks ago and they couldn't see anything at all, so they scheduled me to come back.
Two weeks later I go back and all they can see is a gestational sac. So they asked that I return in a week.
I went back today and they can still only see the sac. The sac is measuring only 3 days further along then last time.
pregnancy tests are still really dark positives (I still take them often), still getting regular pregnancy symptoms.
Conception date should be around October 2nd-4th. 
Gestational sac was measuring 5 weeks 5 days last time and this time is measuring 6 weeks 3 days
Anyone have any experience with this? My nerves are a mess.