I am not on any contraception and use this app to to tell me when I am due etc. It is normally right, I have only ever been one or 2 days late. It's just weird I'm 8 now.
Honestly, I you're on the pill you should be fine. My period came like a month late after having and sorry for the tmi but a lot of unprotected sex. I also took the tests all negative and it turned out to be just what the doctor called a pill period where you don't really bleed but still get the cramps and ill feelings