clearblue digital opk

Tristan
I started testing on CD9 with the Clearblue digital opk (regular, not advanced) Yesterday was CD12 for me and I got a smiley face...and today I tested again and got a blank circle. Last month I got smileys for 2 days beginning on CD14. I'm usually pretty regular with a 28 day cycle, so I'm confused as why I got a smiley so early, and no smiley today. 
Has anyone else run into this?