Could be stress ... Could be any number of things, really. I can't really give you an exact reason.
Why not wear a panty liner when you're away from home? And then you just stick a pad/tampon in your pocket and it'll be ready to use when you need it.
Here are a few threads on late periods ... Please use the search function next time, okay? http://www.scarleteen.com/forum/Forum2/HTML/000233.html http://www.scarleteen.com/forum/Forum2/HTML/001435.html
"When I am alone I am not aware of my race or my sex, both in need of social context for definition."
~ Maxine Hong Kingston