Less pregnant than i thought

I keep pretty good track of my period and ovulation. According to my calendar I would be 6 weeks today. Friday my Dr's office called and said my HCG levels were reading as 3 to 4 weeks pregnant, my dr wants me to get my HCG levels tested every 2 days now. I go in tomorrow for my OB apt and I'll get my new reading from Friday. I had a miscarriage in September. We just started trying again in december and I got pregnant right off. But bc of my levels reading less I'm worried ill miscarry again. I have a toddler who is almost 4.