Have you been to the doctor? Hair loss in women is often caused by a change in hormones, such as estrogen or thyroid hormones. Have you started any new medications? Sometimes that can have an effect. How recently did you give birth? I've heard that hair loss is common after giving birth when the hormones suddenly change.
Go see your doctor for a medical work-up. He or she will give you blood tests and may even take a small sample of your scalp to make sure it's okay. Don't let the doctor tell you it's something you'll have to live with until every possible cause has been ruled out. Often, these things can be corrected if the cause is found.