Epidermoid cysts: These are benign cysts that can form on the skin, including the scrotum. They are usually small, hard, and can be multiple. Given their superficial nature and hard texture, epidermoid cysts are a common and likely explanation for the described symptoms.

Sebaceous cysts: Similar to epidermoid cysts, sebaceous cysts are benign and can appear as hard lumps under the skin. They originate from the sebaceous glands and can occur on the scrotum.
Dermatofibromas: These are small, hard bumps that can develop on the skin after a minor injury, typically resulting from an overgrowth of tissue. While less common on the scrotum, they could be considered given the description.
Fordyce spots: Although typically smaller and softer, Fordyce spots are benign lesions that can appear on the genitals, including the scrotum. They might be considered if the lumps are very small and hard.

Testicular cancer metastasis: Although rare and less likely given the superficial and hard nature of the lumps, it's crucial not to miss testicular cancer or its metastasis. However, metastases are usually associated with a known primary tumor and other systemic symptoms.
Scrotal lymphoma: Lymphoma can manifest in unusual ways, including as lumps on the scrotum. Given the potential severity of lymphoma, it's essential to consider it, even if it's less likely.

Scrotal calcinosis: This condition involves the formation of calcium deposits in the skin of the scrotum, which can feel like hard lumps. It's a rare condition and might be considered if other diagnoses are ruled out.
Cystic teratoma of the scrotum: A rare type of tumor that can contain several different types of tissues, including hair, muscle, and bone. These are usually larger and might not fit the "small" description, but could be considered in a differential diagnosis due to their potential to cause hard lumps.