If your period comes early once in a while, it is a normal occurrence and not a cause for concern.
Menstruation can start before its due time because of a change in your lifestyle, intense physical exercise, illnesses, or stress. However, sometimes it happens without a reason, and this is still not necessarily abnormal. So, if you have your period a week early — don't worry. If it happens irregularly all the time, ask your doctor about it, just in case.
Many women have irregular periods occasionally; others have them throughout their lives.
Early period causes include:
- hormonal imbalance
- illness
- taking certain medications
- menopause
- over-exercising
- poor nutrition
- stress
- sudden weight gain or loss
- travel
Your birth control method can also affect your cycle: an emergency contraception pill can cause an early period. If you skip a few COC pills, hormone levels drop and bleeding may start before its due time. If you have recently had an intrauterine device installed, your body may react to it by bleeding.
To avoid unpleasant surprises with your period coming early, you should always have feminine hygiene products on hand. If you have your period early for a prolonged period of time, ask your doctor about it.
http://www.mayoclinic.org/healthy-lifestyle/womens-health/in-depth/menstrual-cycle/art-20047186?pg=2
http://www.ncbi.nlm.nih.gov/pubmed/16700687
http://www.livestrong.com/article/22195-reasons-getting-period-early/