Early Pregnancy

Okay all you mommas I need help. I haven’t had a period since October 20th. I went to my dr Tuesday (assuming I was around 6 weeks) and they did an ultrasound and we did not see anything. Not even the gestational sac so that made me very worried. They did blood work and my HcG level was a 252. I came back Thursday for lab work and my HcG level was a 641. The doctor said it’s good that it is doubling but it’s still low. Should I be worried? I had a miscarriage in June. I don’t want to go through that again!