Sudden start of back pain during menses is usually associated with some reason but as you have visited the doctor then I would not like to go in details about it. I can help you with certain routine advise which will help you to deal with issue like You should try doing simple exercises with related to abdomen and general body. You should hydrate your self properly. Avoid eating outside, oily, spicy and junk foods, try to eat more of fiberous food like fruits and salad. U can take mild antispasmodic med when your menses are about to start to control pain. Please try this stuffs as they have been useful in no of cases and let me know how u felt. Take care and feel free to contact me in future