According to health writer Rachel Gurevich for About.com, a period that is shorter than usual can be caused by minor health disturbances, such as interrupted sleeping patterns, bouts of high stress, excessive exercise or traveling. Significant weight fluctuations or illness, such as the cold or flu, can also cause short periods. A shorter period can also indicate that a woman is pregnant.
Melanie Winderlich of Everyday Health explains that in the worst cases, a shorter period can indicate that a woman is experiencing a more serious underlying health condition, such as a thyroid disorder or polycystic ovary syndrome. Usually, a shorter period means that a woman did not produce as much estrogen as usual, which leads to less of an endometrial lining to be shed during menstruation.
Girls who have just entered puberty also commonly experience erratic period lengths while the hormones in their bodies are balancing and finding an equilibrium. During the stages leading up to menopause, a woman's body begins producing less of the hormones estrogen and progesterone, which causes shorter and shorter periods until menstruation stops altogether. Because of the menstrual cycle's dependence upon hormone levels, birth control can also be a cause for fluctuations in bleeding times.
Winderlich states that women experience different period lengths from one another, and it is possible for cycles to vary from one month to the next. The length of a woman's period is usually between three to seven days. If a woman is used to bleeding for five days a month and suddenly experiences a month where she only bleeds for two or three days, she should consider other factors in her life which changed around that time.