Armpit Pain for a month. Much worried.

Lesion with pain can be because of anything that is causing an inflammation . Which can be cancerous as well as non cancerous lesions . Most probable cause of painful swelling in armpit , which increase and decrease in size are sebaceous cyst , which are not cancerous , but need treatment with antibiotics or may need removal . Breast self examination is actually suggested by many , but haven’t proven in studies, to have reduced the risk of malignancy . You have to get examined by a doctor , and do a breast mammogram or USG breast scan (since age is less than 40) as indicated. Wound like to know if your are having any other symptoms associated with it , like fever , itching or any discharge . And also need to see exactly at what point you are having this pain , and it’s reliving and exacerbating factors .