Mannnnn i hate always having slightly irregular periods. I have the period tracker app & every month…

Mannnnn i hate always having slightly irregular periods. I have the period tracker app & every month ill check and it'll be like "3 days late" so I panic, buy a pregnancy test only for my period to start hours or a day later. Its really odd because I used to be pretty on the dot with the tracker app, and im not on birth control or anything so I don't know why its like this?