fertility changing

Whenever my period dosent start the day the app says it's going to my fertility moves. For example I have skipped my period for a whole month. While I'm looking on the whole over view I noticed my fertility colors moves down a week and a half from where it was. I'm not sure how I feel about that because how am I supposed to know if it's accurate or if it's going to affect next time. Does anybody else have this problem?