Fertile Week

I have been using Glow for a little over a month now. I have started keeping track of my BBT.  On the app it had my fertile week a week ago. Then it changed to this week.  I am not sure if there was a glitch with the app or if the fertile week changed based on BBT. I am an not sure if this makes sense but if anyone knows wht I am talking about any help would be appreciated! Thanks!