Honestly, most periods are pretty irregular. You may think you got it all scheduled out and everything then BOOM! it gets all weird. Because your eating/sleeping schedule can mess it up. If you’re too stressed that can mess it up. Lots of things can throw it off.
SOOO (There’s a point to this) Getting you’re period a bit late is normal, or early, and if your ovulating 8 days before it could mean it’s early. (or maybe late) I think it’s recommended that girls mark every day they get their period, so they can try and expect it. But for some people it’s so irregular and weird that’s impossible