If you are young, it is not unusual for you to skip a period or two because your hormones are not balanced out yet. You could have not ovulated, or if you are sexually active, it could be pregnancy.. It could also be stress, your diet, a big impact on your life (I.e. Moving, traveling, flying), many numerous reasons, honestly.
Most people think that their period should come on the same day of every month, but it could be a couple days before or even a few days of when you had your period the month before. but it varies with every woman. It also depends on your cycle length, you could have a 28 day cycle or you could have a 35 day cycle. I suggest tracking every period, mensus, and sexual inter course on a calender, each month.
How long have you had ur periode? Are you sexually active at all? If so is it when you r on ur periode? If you just started kbow that ur periode dosnt always come on time it may come early or afew days late
I hope this info helped yours truly ~ xana