No baby found in ultrasound 7 weeks 5 days pregnant

I just had an ultrasound done today and there was a gestational sac, but they couldn't find the baby. The dr. Ordered a quantative hcg test and my levels were at 8200. Seems kinda low to me. They tested me March 9th and they were 4000. It took 8 days for them to double. I am freaking out right now and don't know what to think!! I have another blood test in 4 days and an ultrasound in a week. I can't handle this