Hi. Thanks for your query. Noted the history and understood your concerns. Lump in the left armpit that has persisted for a month, is of the since of a nickel; purple and fairly painful can be due to the following reasons: Dermatitis with skin changes. Localized hidradenitis. Skin cancer This can be diagnosed by the following ways: Get a clinical evaluation and examination by a General Surgeon so that a proper diagnosis may be done and relevant investigations done to see the cause and plan a proper management. Till then apply liquid povidone iodine locally. Keep the area dry and clean. Avoid exposure to hot and humid air.