Worried😐

Devon
So I concieved on December 7th which should make me about 8.5w I'm not sure when my last menstrual cycle was, somewhere around thanksgiving, but I do know my exact date of conception. When I went to the dr thursday 
 he couldn't find a heart beat but she saw the sac. She was measuring the baby to be about 6w4d which doesn't make sense. Except that's the exact time from conception. But I know we start week 1 the first week of our last period should I be worried? Is the baby not growing properly? I haven't had any bleeding or cramping. Get all my vitamin as and nutrients only thing is I've had a lot of stress. She took my blood levels and I go back next week to see if we can find a heart beat but this is all that's been on my mind and I'm not sure if I should be worried!