worried...

I'm 5 weeks pregnant and I went to the doctor. He showed me my uterus and all that. There was no mass or anything. 
But he saw the sack but with no baby. So he has me scheduled the come back in 2 weeks for another ultrasound because he said that's when we should see something. 
Honeslty I'm worried because my cousin in her first pregnancy, she was 5 weeks pregnant went to the doctor and saw a sack but no baby so the doctor made her come back at 8 weeks pregnant. Well she went back and they still saw the sack and no baby, turned to find out she had a miscarriage. 
Has anyone else went to the doctor their 5th week and just saw the sack but later on saw the baby?.... I'm actually worried.