With that said, it is not entirely uncommon to start a period two weeks early. There have been cases where a person will have their menstrual cycle every two weeks like clockwork instead of every month, and many women will get their period early occasionally. There can be any number of reasons, birth control, smoking, substance abuse, sexual activity, pregnancy etc.
One more serious cause which should certainly be checked by your doctor or gynaecologist is ovarian cysts. These have been known to cause a number of menstrual problems, and many personal reports claim that ovarian cysts are linked to their irregular cycles. These can lead to more serious problems, and are best caught early.
If you are sexually active, and have taken the morning after pill, it has been known to have unusual effects on your menstrual cycle. Factors such as this are worth considering before you begin to panic, but regardless of the circumstance, you should still visit a medical professional. It is likely your period will return to normal in time, but never pass up getting problems check out just because of worry.