What does this mean..

So I was spose to come on my period on the 17th Tuesday, I never came on so I pressed to bottom at the bottom that says "my period is late"
The pink them changed to period starting on the 20th (today) ( still not period)
Why would the app change the day of my period?
I normaly have a 27day cycle,
At the moment I'm on cycle day 31?