If you're in the UK go see your local Brook Adviksory service, or your family planning clinic. They have these at schools and colleges too. You didn't mention if you're on the pill too, but if you puke within something like 3 hours of taking the pill for that day, then there's a good chance of it not working. Stress can also make ur period late, as other posters have suggested. Good luck hon.
Don't dream the dream, be the dream....