Low HCG

So I got my BFP on February 5th. I scheduled an appointment right away and they wanted to do a blood draw to confirm my HCG levels amd get a estimation on how far along I am. When they called me with the results on Friday they said that my HCG was pretty low and so they wanted me to come back in after the weekend for another blood draw. What could cause this? Do I have reason to worry? I am trying to remain hopeful that maybe I am just not as far along as I thought I was...I just need a lot of positive vibes and PRAYERS PLEASE!❤🙏🏻🙏🏻🙏🏻