There are a variety of different reasons that a woman's period may start early or even late. In most cases the reasons aren't serious and the woman has nothing to worry about.
The most common reason that a woman begins her period early is because of significant stress that she may be feeling. Stress can affect the levels of a woman's hormones (oestrogen), which then affect the timing of her period.
Drinking liquids such as coffee, tea and soda can also cause the woman's period to start early. Caffeine can cause irritability, nervousness and sleeplessness in women. This may also upset the timing of a woman's cycle.
The reaction your body has from starting or changing the type of birth control you use can also affect the period. This is similar to any other type of medication you first start to use. Birth control can cause irregularity in your period for the first few months of use. It also tends to lighten your period.
Hypopitutitarism is the most serious cause of early periods. If you have Hypopitutitarism it can cause your pituitary gland to slow down. That means, it won't produce enough of certain hormones. This can cause major effects on your cycle because this gland produces six major body hormones. Your cycle can be drastically affected. Consult a doctor if you are concerned about this.
Every woman's period is different. A girl is born with a number of eggs and when one matures an egg will ripen each period of menstruation. During this time, the hormones in your body will circulate. The egg will mature and fall from the ovary into the Fallopian tubes and the hard lined uterus. If the egg is not fertilised it will break apart and the uterus will shed it's lining. Because there are so many different factors that go into a menstrual cycle, it's normal for the cycle to vary.