HCG levels doubling but nothing seen on ultrasound. Help please

Courtney
I could really use some success stories. My HCG was 931 on Sunday then 3026 on Wednesday. But they can't see the sac or anything in uterus. They thought maybe ectopic but they didn't find anything. 
Anyone have similar experience where there hcg levels were over 2000 and they didn't see anything but then later they found it?
Could use some hope