Dear patient thank you for that question. The problem I have with answering the question is that there are so many reasons that I could never get through the entire list. So I shall try and explain the most likely and most dangerous ones and the rest I shall list under a broad heading while telling you what to do about those too. 1) Angina: It results in a squeezing sort of a pain. My reason for mentioning it first is that is dangerous and easy to rule out. Just get an extra done and a troponin level done 2) Breast cancer: Swollen lymphnodes can result in such pain which could either be due to an infection or the tumour has spread to your lymphnodes. Get a mammogram specially if there is a family history of Breast cancer 3) Pleural pain or pleurisy: Inflammation of the membrane covering the lungs can result in such a type of pain. Get an x-ray and see if taking Ibuprofen 800mg helps. A Charleston of Pleural pain is increase in inspiration 4) Neuropathy: Classically a shooting sort of pain which runs down the arm. it is due to the involvement of the brachial plexus and a nerve conduction test may rule it out 5)Anxiety is often associated with underarm pain since it results to rapid heart rate, shaking, sweating and irritation; all which might cause underarm pain in some way i.e. rapid heart rate and shaking strain chest muscles and making them feel achy while sweating will cause irritation which can cause burning and itching (burning and itching can make your underarm painful. 6)Local glandular infection: Stand infront of a mirror. Raise your arm and touch the armpit to look for any increase in temperature or any local tenderness Discuss these with your Gp and come to a diagnosis and again I cannot stress enough that please rule out Angina and Breast cancer I hope this answers your question