Menstrual cycle is a biological process which initiates in a woman after puberty. It is a character of womanhood, which decides fertility in the women. Menstrual period normally lasts for a period of three to five days, during which blood and fluids discharge from the vagina. The average age at which girls start menstruating in the US is 12. But it varies between 8-16 years.
The symptoms for the onset of the menstruation are:
Cramping, bloating and sore breasts
Food cravings
Moods swings and irritability
Headache and fatigue
Premenstrual syndrome (PMS) is a common ailment among the women who menstruate. It occurs due to the hormonal imbalance one or two weeks before menstruation. It causes problems such as depression, fatigue, allergies and hair loss.
Its better exercising for good health but overdoing may cause problems. If the PMS persists, one may seek the help of a trusted adult or else a doctor.
This cycle occurs with a span of 28 days. It switches between the two fallopian tubes where the ovaries develop. The first five days is the period of menstruation. The fertile time is between 11-18 days.

Source: womenhealthzone.com