opk newbie

Shea • 29* 3 Awesome sons*🌈 baby🌈 #4 on the way💜

This is our first ttc cycle. I am currently cd 15. This is my second cycle after ending BC so I'm not exactly sure when I ovulate. According to my app it should be on cd 17. This am I had a drop in bbt from 96.7 to 96.5. I took a Dollar Tree opk at 5:30 this evening,and there was a extremely faint line. Ive never used an opk before and I am a bit confused....