Help My dates changed!

Was getting into app to log in today, write symptoms,exercise, sleep, weight etc ....and noticed my period due date had changed! Yesterday it said in 11 days and my partner & I had sex during fertile window. Now today it says period in 15 days!!!! And my fertile window changed too!! Now what!? How did this happen!? Now it's easy confusing when to be sure to have sex. And if we even did it on the right exact days! I just don't knew why it changed all of a sudden and how I can fix this! Never happened before and I thought I was tracking everything well. #SoConfused