Late period

So my periods late. I was supposed to start on the 20th but I didn't, i pushed the button on here when you check your period calendar and see when your next one is so I hit "my period is late" and it jumped to the 22nd which is today... And I still haven't started it. But I told my friend about it and she asked if I had been more active now than I usually am and i said yes... And she said that it was just thrown off balance. I was just wondering what else could throw my period off and if I should expect it not to come at all till my next period.?