Muscle Strain: This is often the most common cause of armpit soreness due to overuse or injury of the muscles in the shoulder or arm, such as during sports or heavy lifting.

Axillary Lymphadenitis: Inflammation of the lymph nodes in the armpit, which can be caused by infection, often presenting with soreness, swelling, and sometimes fever.
Shaving or Waxing Irritation: Irritation from shaving or waxing the underarm area can lead to soreness, redness, and bumps.
Deodorant or Antiperspirant Allergy: Allergic reactions to ingredients in deodorants or antiperspirants can cause skin irritation and soreness in the armpit.
Breast Cancer: Though less common, breast cancer can sometimes present with armpit soreness due to lymph node involvement, especially if there's a palpable mass.

Lymphoma: A type of cancer that affects the immune system and can cause swollen and sore lymph nodes in the armpits, among other symptoms.
Breast Abscess: An infection of the breast tissue that can cause severe pain, swelling, and soreness, potentially extending to the armpit.
Cat-Scratch Disease: An infection caused by the bacterium Bartonella henselae, transmitted through cat scratches or bites, which can lead to swollen and painful lymph nodes, including those in the armpit.

Axillary Vein Thrombosis: A blood clot in the axillary vein, which can cause pain, swelling, and soreness in the armpit, often associated with arm swelling.
Lipoma or Other Soft Tissue Tumors: Benign tumors that can grow in the armpit and cause soreness, especially if they press on nerves or other structures.
Hygiene-Related Conditions: Poor hygiene can lead to conditions like intertrigo (a fungal or bacterial infection in skin folds) in the armpit, causing soreness and discomfort.