Why can't I see my fertile days & percentage?

I use an IUD for birth control, however, if I select IUD in the app, my red fertile days & percentages disappear on the calendar! I have to select another BC method for the days/percentages to come back. Why? I know that with an IUD, I shouldn't worry about pregnancy, but I still like to avoid sex on my fertile days just to be safe. Nothing is 100%, and I really, really don't want children!