Why did my fertile window change?

birdrogers • I have a dog & a husband. If it weren`t for them, I would have 17 cats & live alone.
I just got off my period yesterday. Today I got an alert that I am in my fertile window. The last few months it has been on cycle day 11 or about a week after my period, but now it's cycle day 5. 
Which is it and why did it change?