HCG level 6 at 4 weeks or just a little shy of 4 weeks

I had a blood test today. Per due date calculators I should be 4 weeks (but maybe less because I have longer cycles 29-35 days) and I had HCG level of 6. They say this is low and could be due to the early state of the pregnancy or could be something bad. Has anyone experience low hcg levels at this point in pregnancy? I'm scheduled for a follow up test Thursday (in 2 days) to see if the levels have increased at least by 70%.