fertile window changed...whats up with that?!?!?

Ashley • Hi!! My names Ashley Im 34 and have 3 daughters and im madly in love with my husband my whole fam has baby fever😀
im new to all this and my fertile window was last week then i just got on today and it has changed before my most fertile day was sunday now its sayin it was yesterday which is okay cuz ive been bd almost everyday but why does it change????