Need help - First Response OPK

Dae
Glow predicts that my fertile window starts today. Ovia predicts it doesn't start until Tuesday. I am CD 13 and using First Respose OPK for the first time. 
Based on my average cycle (34 days), I wasn't supposed to start testing until CD 17, but I decided to try today. It looks like it's positive to me today. Could I be ovulating already?