Possible Twins!!

So lm really worried got my ultrasound about a week ago and the doctor showed me a baby with heart beat and than they moved over a little and they seen a second yolk sac with no baby so I wonder if the yolk sac has not already develop does that mean it’s a high chance I will lose the other baby I was 6 week and 2 days the doctor said it was too early to tell but that we will back 2 weeks to see what develop I just been wrecking my brain every since ....