period late even though i haven’t had sex


my last period was from march 1st to march 6th. my first period was in 2015 in august but im pretty sure my periods have started to become regular. my period is 2 days late according to this app and 5 days late according to another app. i have recently started working out after not working out since last summer but the workouts aren’t super strenuous (just lifting around 60 pounds, using the treadmill at a pace of 3 for 2 miles, and lifting 120 pounds with my legs). im a virgin so i know i’m not pregnant. i also have been stressed out from school so that could be another cause.