LUCY 2007.
Hi. You don't specify your age. If you're menopausal, or post-menopausal, hormone loss could be the answer. Some medications can also cause hair loss. If you've not yet reached menopausal age, then some of the possibilities could be eating disorders, medications, too. Stress is another factor, as is a genetic disposition (family history). Have you checked with your doctor? A dermatologist might be a good doc to start with, and perhaps get some blood work done, too.
Best of luck