Fertile window help?

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Hi ladies ! In my home screen where it has the calendar circle and period days are red, it says my fertile window days will be green, but I have no green days ? Is this because I am on the pill and put that info in when I joined ? Also if so can someone explain when in the month you ovulate in relation to your period? And also when you're most fertile each month? I'm 24 and should probably know this stuff but I don't !