Your daughter is demonstrating signs of trichotillomania. My niece was diagnosed with this as an infant, and I noticed that my 9 month old was possibly starting to pull her own hair several months ago. It is chiefly associated with anxiety and/or self-soothing, but can be controlled if you act now. First of, definitely speak to your pediatrician. S/he may suggest that you cut or buzz her hair, something I am sure you will not want to do, but my brother and his wife cut my niece's and it was the best thing for her. Since she was an infant, she did not understand NOT to pull her own hair. This was the extreme solution but the most successful in the end. Second, a fuzzy, fringey blanket to play with and cuddle or something silky will help. Third, try to resist saying "no" when she does it in front of you. Instead, just silently move her hand away from her head and place another object (like the blanket or fingey toy) in her hands to keep them occupied.
My niece is now 7 years old with a beautiful head of hair to the middle of her back....no pulling at all. And my daughter still puts her hand to her head and gets a small amount, but knock wood, she hasn't actually pulled. I think the toys and blanket have helped. I hope you find that this works. Best of luck!