Epididymitis: This condition, characterized by inflammation of the epididymis, often presents with sudden onset of pain in the testicle, which can be accompanied by swelling and redness. The rapid onset and the presence of a nodule could be indicative of an infectious or inflammatory process.

Testicular Torsion: Although it typically presents with severe pain and swelling without a distinct nodule, it's a possible diagnosis if the nodule is interpreted as swelling or if the torsion is partial. The key feature is the sudden onset of pain.
Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections, can present with pain, swelling, and possibly a nodule if an abscess forms.
Varicocele: A varicocele is an enlargement of the veins within the scrotum, similar to varicose veins. While it's more common on the left side and can cause a sensation of a nodule or swelling, pain is less common unless it's a new varicocele or it's thrombosed.

Testicular Cancer: Although less likely to present with sudden pain, testicular cancer can cause a nodule. It's crucial not to miss this diagnosis due to its potential for serious outcomes if not treated early.
Acute Hydrocele: A collection of fluid around a testicle can cause swelling and potentially mimic a nodule. While usually painless, an acute hydrocele can be painful if it develops rapidly.

Spermatocele: A spermatocele is a cystic structure filled with sperm that arises from the head of the epididymis. It can present as a nodule but is typically painless.
Cyst of the Epididymis: Similar to a spermatocele, these are cystic structures that can form on the epididymis and may be felt as a nodule. They are usually benign and painless.
Henoch-Schönlein Purpura (HSP): A systemic vasculitis that can rarely cause testicular pain and swelling, potentially mimicking other conditions listed here. It's more commonly associated with skin, joint, and gastrointestinal symptoms.