When I first started <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> it calculated my period to come on the16th  and my fertile window from May 28 -June 3 however Af is not here ambit I took 2 test both negative.. So I went to my calendar to put my period was late and it changed everything which means my fertil window was wrong😩 I'm just so confused.. So is my period really late or did glow calculate wrong ?