Period a week early could mean what?

Normal variation: Having your period come a week early can be a normal variation. Changed in mood, stress, contraceptives can all alter your cycles. I would continue to observe a trend in your cycles.

Early period: Usually this just means you had a shorter cycle than normal. It is not unusual to have fluctuations in your cycle lengths while in young adulthood. However, it can also be a sign of early pregnancy if you have had unprotected sex in the last month. Check a pregnancy test to be sure that's not the cause. Weight fluctuation, stress, the weather, and strenuous exercise can also be causes to consider.

Lots of things: Irregular periods could be due to stress, exercise, diet changes, medications, to name a few possibilities. I would recommend you wait a couple of cycles to see if this is a pattern. If it continues, or if you have pain or other symptoms, see your doctor. Good luck!