Are you basing your 5weeks on ovulation or LMP? At the beginning of my pregnancy, I thought I was 7 weeks along (based on LMP, but I O late in my cycle). I went to the doctor for cramping issues, and they did an US. There was a sac, but no visible baby/hb. I had my HCG levels drawn, and they were at 3000. The doctor said I could be miscarrying. They wanted to see me back in two days for another HCG draw. My levels went up, but barely doubled. They adjusted my 7 weeks to 4weeks, and did another US a week later, and saw a wonderful, healthy heart beat. I now have a very happy and healthy 4 month old.
Please try not to worry, I know it's hard, but wait and see if your levels go up!