Adjusting after B.C.

So I came off of B.C. In June 2021. I was using nuva ring. My cycle has always been regular and stayed regular after the first month being off B.C. Since august the app has been wrong I predicting my period by almost a week. My period still comes on it’s on, it’s still the same amount of days, and there has been no change in symptoms or pain. Any ideas as to what could be causing that? It worries me because if the period is later than predicted with the app then that means ovulation time is off . Any suggestions?!