No real reason. Maybe you miss counted and if you keep a calendar check that . If you are young and just starting a period it will be irregular for a year or so.
depends on how late. You may be pregnant, but it can also be caused by stress and inadequate nutrition among other things
Nothing. Your period starts a fixed 14 days after your ovulation day.
Once in a while your cycle can go a bit astray however it usually returns back to normal before long. It is nothing to worry about.
Yes, day 1 is the first day one starts bleeding.