two apps, different info!!

Okay, so, I have been using a different app to keep track of my periods and stuff and it has been pretty accurate with predictions of when my period was going to start. I just went to my doctor a little over a month ago and she told me about glow. This is the app she uses and just had her second baby with using it. The only probably I am having is that the apps are a week off for everything!! My fertile days and my period!