Blood Test showed low progesterone levels??

A
I'm trying not to freak out because my doctor said there's no reason to but I had a miscarriage on March 2, so I'm nervous. 
I had blood work done on Tuesday (4w4d) to test HCG and Progesterone levels. My HCG levels were good, 173. But my progesterone level was at 13.5, when doc said they like it to be at 15 at this point. 
She proscribed prometrium for the first trimester. 
Has anyone else taken this?? Or had low levels?? 😩😩