Erythema nodosum: This condition is characterized by the sudden onset of tender, erythematous nodules or plaques, typically on the lower extremities. It is often associated with an underlying infection, inflammatory disease, or drug reaction, making it a common cause of indurated and intermittently tender skin lumps.

Lipoma: Although typically not tender, lipomas can become inflamed or irritated, leading to intermittent tenderness. They are common benign tumors of fat cells that can feel like indurated lumps under the skin.
Epidermal inclusion cysts: These cysts can become inflamed, leading to tenderness and an indurated appearance. They are benign cysts that can occur anywhere on the skin.
Furuncles or carbuncles: These are infections of the hair follicle or skin that can present as tender, indurated lumps. They are caused by Staphylococcus aureus and can be recurrent.

Squamous cell carcinoma: Although less common than other skin cancers, squamous cell carcinoma can present as an indurated nodule. It is crucial to consider this diagnosis, especially in sun-exposed areas or in individuals with risk factors.
Merkel cell carcinoma: A rare but aggressive skin cancer that can present as a firm, painless nodule but may become tender. Early diagnosis is critical for treatment.
Cutaneous lymphoma: Certain types of lymphoma, like cutaneous T-cell lymphoma, can present with skin lesions that are indurated and sometimes tender.

Panniculitis: Inflammation of the subcutaneous fat that can cause tender, indurated skin nodules. It has various causes, including autoimmune diseases and infections.
Necrobiosis lipoidica: A rare condition characterized by necrotic, granulomatous lesions in the skin, often on the lower legs. Lesions can be tender and indurated.
Sarcoidosis: A systemic disease that can cause skin lesions, among other symptoms. Skin manifestations can include indurated plaques or nodules, which may be tender.