HCG levels didn't double...

Did anyone at a certain point not have their levels double? 
Last week I went in and they only saw a sac. I was suppose to be 7 weeks but doctor said maybe I was earlier. 
So Thursday's beta was : 16000
Saturday's beta was:  20,500
Not doubled... 😬... But I've read that after a certain level it takes longer for it to double. Not sure what to think right now