so I was supposed to start my period yesterday!

So anyways.. I'm confused! Glow was saying how my period was going to start yesterday, but it didn't. So I charted that on the app and instead of saying I'm late it just changed the date of my period! Saying now I won't start for another 3 days! So what does that mean I'm not supposed to start until then now?! Lol This app just made me mad. also I'm on day 42 of my cycle.....