Maybe this is why

I just downloaded the glow app and had previously been using my period calender. The fertile days on each app are different and the period start dates were different. Old said I should start Thursday and glow said Monday. I got AF today :( so I guess glow is more accurate. So I should follow the fertile days on glow instead, maybe that's what I did wrong with the other app. I just always thought you ovulated 14 days after you start your period.