Question about a change in fertile window

So glow was telling me that my fertile window was last tuesday through today and that my peak day was Saturday. So on sunday morning when I entered my log for the day Saturday and saved it changed my week 2 days ahead and said my fertile window was last Sunday through sat. Which screwed me up for my peak day. Did I just waste another month by missing the right day? Does glow automatically change your window based off of bbt records? That's the only thing I could think would have made it change.