AF was suppose to be 6/18.. Now it's 6/21.. I don't get it!!!

My AF suppose to come on 6/18 this month.. I'm usually on time.. Give or take a day.. But now I'm late going on two days.. I'm kind of nervous now for some reason.. So I logged on my AF chart that it didn't come and now it says it should come on 6/21.. Which is sort of confusing because I'm not understanding how Glow even got that date.. Anybody else AF is on these days???