There are different types of fat in the body - essential fat and storage fat. The essential fat is what makes up your breasts and little bits in the muscle etc. The storage fat is the excess fat that can build up on your stomach, bum etc. Women in general need a little more fat than men as it ensures they have enough energy stored up for having a baby. While this may not seem necessary in this day and age when food is so readily available, during times of starvation and famine it is absolutely essential that a woman has enough fat on her before she can get pregnant.
To ensure this is the case, the body shuts down its reproductive system when it does not have enough fat stored on the body. While some girls think this is a convenient way of preventing a pregnancy, it is not. The hormones that would normally circulate through your body to instigate and regulate your periods are essential for other areas of your body as well. Girls who do not get their period regularly due to being too skinny put their bones at risk of becoming brittle i.e. they can develop osteoporosis.
Low levels of circulating oestrogen can also lead to a loss of elasticity in your skin, resulting in premature aging; as well as a drop in your body's good cholesterol and rise in its bad cholesterol.
I would recommend you have a chat to your doctor about what you can do to maintain a body shape and weight you like, while being in the healthiest shape possible for the rest of your life.
Better Health Channel: Menstruation - athletic amenorrhoea (www.betterhealth.vic.gov.au/