ultrasound, early pregnancy.. 5weeks 4/5 days

Hi, could someone please help me out.
I went to A&E five days ago with pains, yesterday I had a early scan they can see the sac but nothing inside it, my hCG levels are 16,000 and roughly 5 weeks and 4 days
Ive got to have an internal scan tomorrow.
Ive had no bleeding just cramping on both sides and in the middle of my belly, Ive had a few sharpe pains on my right side but no other symptoms, my levels have doubled but have now stopped. I also feel so poorly,  I'm absolutely knackered and feel drained!!
What does this mean?
They don't think it's a ectopic pregnancy, and no sign of miscarriage.
Is it just to early to see baby? Or is it a delayed miscarriage?