HCG levels

Hi! I am 6 weeks today. I've had my blood drawn several times and my numbers are not doubling. I was 298 went to 450 then to 897 (there were 5 days between those two). I just had my blood drawn again today to see if they've gone up, and have an ultrasound scheduled for tomorrow. So my question is, has this happened to anyone? If so, what was the outcome?  I'm trying to not stress and stay positive but it's getting hard. Thanks!