Most fertile days

This is my first month using glow, so it's possible I'm not getting perfect predictions. I'm on my last day of my period and glow is saying the next 2 days are my fertile days. I use another app, woman log pro, and that one always says 10 days after AF. I've been temping and am pretty aware of my CM. The thing I'm most curious about with TTC is the luteal phase. I don't always get 10-12 days from the suggested O days until the next AF. I bought the 20 day supply of OPK, should I start tomorrow? When do you normally start checking O after AF? (Hoping I used the right abbreviations!)