period

So ladies? On my <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> it said I was suppose to start my period on the 18th and it did not come. Than I logged it and the date for my period changed to the 20th which is today and it has not started yet! But when I log it as it hasn't started yet it changes the date to the 22nd!? I'm so confused when should I have started my period! I'm ttc and pregnancy tests are coming up negative. Should I take another one? Or wait. Please help!!