Soooo

Okay so I got my ultrasound today and I was thinking i was 7 weeks but the lady said she was concerned about the baby growth but then she said im still to early too tell how far along i am ...when I went to the 1st ultrasound it was just a sac and she said I was 5 weeks this time something was in the sac so my question is can I actually be 5/6 weeks now idk why they saying they don't know how far along I am.