Back pain

It gets worse during my period but I have really bad constant back pain and my doctor does nothing about it. I’m 15 and she just writes it off as weight from my back pack, stress, period, etc. during the summer I have back pain despite not wearing a back pack? During my calmest moments I have back pain? I always have back pain. She won’t listen to me and it hurts so bad it’s painful to walk. I do yoga and it doesn’t help me. I’m afraid if my back pain can’t be helped I won’t be able to do tennis this year. What should I do? Medicine (ibuprofen and stuff) won’t work, nor does going for walks, nor does resting. Nothing seems to help and the pain is so bad I feel like crying. Something is wrong with me but I can’t find answers.