Musculoskeletal strain: This is often the most common cause of armpit pain, resulting from overuse or injury to the muscles or tendons in the shoulder or arm, leading to strain in the axillary region.

Axillary lymphadenitis: Inflammation of the lymph nodes in the armpit, which can be caused by infection, often presenting with pain, swelling, and sometimes redness in the armpit.
Breast cancer: Although less common, breast cancer can cause pain in the armpit due to metastasis to the axillary lymph nodes, especially if the cancer is located in the outer part of the breast.
Shingles (Herpes Zoster): Reactivation of the varicella-zoster virus can cause shingles, which may present with pain in the armpit area if the nerves in that region are affected.
Cyst or abscess: A benign cyst or an abscess in the armpit can cause localized pain, swelling, and sometimes discharge.

Axillary vein thrombosis: A blood clot in the axillary vein, which can lead to severe complications if not treated promptly, including pulmonary embolism. It presents with arm swelling, pain, and possibly discoloration.
Lymphoma: Although rare, lymphoma can cause axillary lymphadenopathy (enlargement of the lymph nodes in the armpit), which may be painful.
Metastatic cancer: Cancer that has spread to the axillary lymph nodes from other parts of the body can cause armpit pain and should be considered, especially in patients with a known history of cancer.

Cat-scratch disease: An infection caused by Bartonella henselae, transmitted through cat scratches or bites, which can lead to axillary lymphadenitis.
Hygiene-related conditions: Such as hidradenitis suppurativa, a chronic skin condition that can cause painful abscesses and nodules in the armpits.
Neurological conditions: Like neuropathic pain syndromes or nerve entrapment, which can cause pain in the armpit area due to nerve compression or damage.