app changed my start date

So I used this app when I was trying to get pregnant with my son and it was great! So now we are trying for baby #2 and I am losing my mind. Af was due yesterday and nothing and same for today so far... when I put in that my period is late on the app it updated and moved my period start date to Wednesday?... Has anyone else experienced this with this app? I tested today and got a negative but I don't feel like I usually do when my period is about to start. Does this mean all my days were off this month and I may have missed out on my chance of getting pregnant?