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Is there any swelling in that region. Is this the time just before your menstrual period. You could be having inflammation of that region of the breast called Axillary tail of Spence. You can take a pain killer like Paracetamol
for trial of pain relief. Avoid touching or rubbing the area. Don't do excessive arm movements, give rest to right arm. Apply warm compress with water bag for relief. Consult your Doctor if the pain is not relieved or you find any lumps.