Where the OPK pros at?

Click for full picture. I go back and forth between taking 2-3 opk's :> because I have no clue at all when my ovulation is. No worries. I bought like a million! Anyways. The bottom is getting overall progressively darker. And this is my fertile window according to Glow. Is it normal to have it get this dark but go back to very faint(mainly only referring to the bottom)? By the way I started testing the day after my period ended to assure I had good chances of catching it.