Well, if you haven't had sex, you aren't pregnant, and don't worry, periods can shift around and be irregular for any female. It is not uncommon at all for this to happen to someone who has only been getting her periods for a couple of years. Even women who have an established schedule will sometimes find they change and cycle along with other women -- for example, college roommates who all move in together sometimes discover that they begin to cycle together. Don't worry about a change of schedule. Write things down on a calendar or use an online calendar, and if the irregular schedule continues for several months, ask your doctor. But otherwise really, don't worry.