Ovulation Window

So, I've been keeping a close eye on my ovulation window and last week the <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> showed that I would start ovulating on Tuesday (11/15). I checked the app in the afternoon (11/15) and logged a positive  OPK test. This morning (11/6) I went to update my log after my hubby and I did the BD and my fertile window has completely changed. Now it says that I'm already half way through it. I'd hate to have missed this cycle. Has this happened to anyone else??