Period Day 1 was 15th sept....

Using clear blue digital opks from day10 (25thsept).....all opks leading up were negative until a Peak on the day15(30thsept)

My <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> said I ovulate on day16 because of the test info I put in....but My flow app said I ovulated on day12......I'm so confused...I'm so over it....

Any advice?