First ultra sound questions? Help!

I went for my first ultra sound yesterday and I'm guessing I'm around 5 weeks still waiting for the blood test to come back. But the ultra sound just saw a small sack and nothing else. Is it too early to see anything yet. They said in 1-2. Weeks I should come back again and they will see more? Has anyone had this happen before? I'm afraid of this. It could be nothing and it could be something. Help? Thank you.