You could have a hormone imbalance. You should go to your regular doctor. He or she will do some blood tests and maybe a scalp biopsy to find out what's causing it. Most of the things that could cause it are reversible, such as thyroid imbalance, anemia, certain medications, etc.
Just make sure the doctor does not dismiss you by saying you'll have to live with it or some garbage like that. Make the doctor do a complete work-up.
If your regular doctor can't help you, there are doctors who specialize in women's hair loss. You just have to find them.