Why is my hair falling out?

There are several causes of acute hair loss in the pattern that you describe, but two of them are most likely. The first would be a medical condition leading to hair loss. The two most common are thyroid disease and iron deficiency anemia. These are more likely if you have any other symptoms that go along with these two conditions, such as changes in mood, fatigue, changes in the texture of your skin, weight changes, or heavy bleeding (such as with menstruation). Most healthy people your age who experience hair loss should be evaluated by their primary care doctor for these two conditions with simple blood tests and a physical examination. If these tests are normal, then the next most likely possibility is something called telogen effluvium, which is a loss of hair precipitated by some acute stress. This stress can be psychological or physiologic (such as a recent severe illness, a new medication, or a sudden change in diet or exercise). The treatment for telogen effluvium tends to be supportive, meaning that one waits it out and tries to correct obvious things such as diet. If, however, the hair loss is too distressing to you, you may talk to your doctor about possible medications to treat it.