First ultrasound - empty sac. Hope?

I'm 5wks 3days pregnant. I had an MC in May. Last week I had cramping and bleeding. My doctor put me on progesterone and it seemed to do the trick. My HCG seems to be doubling appropriately. I went for an ultrasound today and all they could see was an empty sac. No baby or fetal pole or anything. I'm hoping it's just too early, but I'm so worried. Has anyone gone through this? Please share your outcome, whether it was good or bad. Thanks!