Help! My poor daughter...?

Recently, my fifteen year old daughter, Martha has been having really bad period pains, and her period has been regular since she was ten, and never hurt her, but now she is bent over her bed in tears. Her twin sister Jasmine, and younger sisters, Saffron (12) and Florence (9) all have their periods, but never had anything like this. She has been to a doctor, and it was no use. How can I help her?