Low HCG

I just had a blood test to confirm my pregnancy and my HCG level was around 380 which meant I was around 4 weeks yet my sister is 6 weeks and had 5000+ HCG level...does anyone know if this is too low?? Doc didn't say anything bad, just to come back in two days to ensure my levels were doubling but I'm still a bit worried. Can anyone shed some light....