Baby's Unknown Location?

My husband and I have been TTC and finally I got the two lines for the first time in 5 years! I took several test out of disbelief and called my Dr after to schedule the 1st ultrasound. They informed me based off my last cycle I would be around 5 weeks 4 days. When they did the ultrasound however, they did not see a sac. I am doing blood work 2 days a part and another ultrasound 1 week from today. I am scared because they said it can mean I lost this one. Is it common for them not to see anything?