Fertile window Question

Dawn
I'm new to this and trying to figure out the app. Why does is it say I'm in my fertile window the day after my period ends?  I thought it was more like 12ish days from day 1? This is more like 6 days after day 1? Is it wrong or should I still listen to what the app tells me?  I haven't done ovulation tests yet, I'm relying on my app and my body to tell me.