Period shorter than usual

Acacia

My period always comes and ends EXACTLY when the app says it will, my periods always come the day it says it starts and ends the day it says it ends, this time I was supposed to be on my period the 13th until the 18th

I got my period the 13th - 16th and it was very light Hardly bled and not cramping or headaches or usual symptoms what could cause this?