Positive pregnancy test but had an ultrasound today and there is nothing there but a thin white line.?

They said im either to early like under 3 weeks or that its a tubal pregnancy or misscarriage. i have not had a period since april 1st so they thought they would see a 7-8 week sac. Keep in mind i have pcos so abnormal periods. Im worried , they rescheduled me in 3 weeks for another ultrasound hoping to see more but as of now what do i think?? am i to soon to see anything or is there a problem?