BBT + Opks

I have started on opks this month, been testing every day since cd11. Tested on cd14 in the morning and it was negative, next morning (cd15) I tested again and it showed a faint line, tested again in the evening and it was negative? My bbt had a rise on cd15 and then fell drastically . Could I have missed it on Cd14 eve or it just didn't happen? Can someone help me please? ​