Ovulation test

Nikki

I’m currently on day 3/cd 15 of my predicted fertile window. I’m taking opk test when I remember to do it, but I did it this morning and it’s not super dark. I’m new to the opk test so I’m not exactly sure how it works but, I do know it’s not positive, however, the line is slowly getting darker. It was a little more faint yesterday evening. Does it still mean I’m in my fertile window or no?