I think I was about 8 weeks when we could finally see something in the sac, but even then it still looked like a little jelly bean wiggling around!
With my last pregnancy at 5 weeks they could only see the sac too. About a week and a half later the baby showed up. So yeah its normal.
Sounds to me like its just too early to tell. Every woman and every pregnancy is very different, so you should wait till the next one before you start worrying too much!
Yes I think it's too early!! Pretty rare when you can see a baby at this stage.
HCG levels are doubling fine = great!