Good progesterone levels but HCG levels have not doubled just a 600 point increase

Hi ladies I just got a phone call from my obgyn about my pro levels being great but at 5 weeks exactly my hcg levels haven’t doubled they only went up 775ml/g. I’m afraid because I haven’t heard of this before has anyone ever gone through this before. I’m worried i am trying not to stress, it’s hard to do when everything comes up as a potential miscarriage. I am now 5 weeks and 2 days pregnant. This is frightening. Has anyone gone through this before?