Confused

Be
I started using ovulation test strips on CD7 and got all negatives initially and my Glow calendar looked like this ...
With the beginning of my fertile period starting on the 17th. Then I tested at 1am on the 17th (CD12) and got a positive. Tested three more times that day and all positive, now my Glow calendar look like this......
The beginning of my fertile period shows as 1/13 (CD8)....moved up by four days!  Any idea why and what this means?  Thank goodness we've been sexing it up pretty much daily. 
It almost looks like I'm ovulating earlier than I thought. Could be why I haven't gotten pregnant.