Which one to consider to know my fertile day?

Ive been logging all possible data in the app and <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> says my 18th day post periods is the most fertile day. But my cervical mucus was egg whitey (which is a sign of ovulation) during my 12th-13th days. Which one to beleive ? ☹️🙄