no you don't need to be worried. When you are that young your period can be really irratic. It takes some women till adulthood to get normal, planable periods. Your good for now.
When you are so young, it's not abnormal for your periods to be abnormal from time to time or even consistently for a while before settling down into a pattern. Not to mention, cycles vary between 21-28 days (sometimes less/more), and you're just barely touching the start of that. If you don't get it for another couple weeks, then have your parent take you to see a doctor to look into either having it regulated (which your parents may be against, if they're old fashioned and don't realize birth control doesn't equal "I'm having sex") or just to have it looked into. It would also help if you got a day planner and marked the days you have your period in the calendars for each month of the year to begin to see what's normal for you and what's not.
Not always it might mean that you have irregular periods but if u don't get your period soon I would have your parents take you go see the doc to help regulate your period good luck!!!