Contact Dermatitis: This is the most likely diagnosis due to the common occurrence of irritants or allergens in the armpit area, such as deodorants, antiperspirants, or shaving products, which can cause itchy, irritated skin and bumps.

Folliculitis: An inflammation of the hair follicles, often caused by bacterial or fungal infections, which can lead to itchy, red, and bumpy skin in the armpits.
Eczema (Atopic Dermatitis): A chronic skin condition characterized by itchy, inflamed skin, which can affect the armpits and cause bumps.
Hidradenitis Suppurativa: A chronic skin condition involving blockage and inflammation of the sweat glands, leading to painful bumps and itching in the armpits.
Keratosis Pilaris: A harmless condition that causes small, rough bumps on the skin, often on the arms, legs, buttocks, and sometimes the armpits, due to keratin buildup.

Lymphoma: Although rare, lymphoma can cause itchy skin and bumps, and it's crucial to rule out this potentially life-threatening condition.
Insect Bite or Sting Reaction: Certain insect bites or stings can cause severe reactions, including anaphylaxis, which is a medical emergency.
Tuberculosis or Atypical Mycobacterial Infection: These infections can cause skin lesions and bumps, and although rare, they are critical to diagnose and treat promptly.

Fox-Fordyce Disease: A rare skin condition characterized by itchy, dome-shaped bumps in the armpits and groin area, caused by blockage of the sweat gland ducts.
Granuloma Inguinale (Donovanosis): A rare bacterial infection that can cause ulcerative lesions in the genital and anal areas but can also affect the armpits.
Sweat Gland Tumors: Benign or malignant tumors of the sweat glands, which are rare and can cause bumps and itching in the armpits.