Glow keeps changing my O day!

Clarissa

When I log daily, it keeps changing my ovulation day. I am a normal 27-28 day cycle, normal periods. Today is cd 15. Not as much ewcm as usual this month. And its my first month using opk and have not had a clear positive. Slight cramping today. But now its saying i wont O til cd 17?! Should I trust this?