AF moved!!!!

My fertile week was supposed to be 12-16th may nd AF to start 4th may, during my supposed fertile week BD everyday, used pre-seed nd imputed all data only for it to change my fertile to 24th to the 30th meaning AF is not due till 12th may!!! Can't understand why it's done that, didn't want to leave anything to chance so just used pre-seed every time we BD and DTD every other day, defo think I am out this month with all the changes xxx