Why does my period information disappear if I unclick the "last day"

I had found this a good app, but I just tried to adjust the last day and when I unclicked the automatically ticked last day, the info I had added about flow disappeared and it changed to a non-period day. This happened every day I unclicked for and I have lost all the info and am not sure what day I started.

TeakLipstickFiend about iPeriod Period Tracker +, v6.1