Periods

I know that a period app is just a guess of when your period is going to show up. I recently downloaded this app and put my last period in and this month, I was supposed to, according to the app, start my period 4 days ago but I’m pretty sure I’m going to start in a couple days because my nipples have been hurting and that’s what happens every time I’m about to start my period. Does anyone else have problems with apps predicting their period but their periods coming later in the month?