Fertile day changes

Hello ladies. 
I am trying to understand the Glow calculations better. 
My fertile period was originally suppose to start next Friday, however, I took an OPK today and got a positive. When I marked it in Glow, why did having a positive today say that my fertile day started the 2/15? Shouldn't it say my fertile day starts today?? 
This is my first positive OPK since my MC in December so I'm really excited!