Epididymitis: This is the most common cause of testicular pain, especially in young men. It is usually caused by a bacterial infection, such as chlamydia or gonorrhea, and is characterized by pain, swelling, and inflammation of the epididymis.

Orchitis: Inflammation of one or both testicles, usually caused by a viral or bacterial infection. It can cause severe pain, swelling, and redness of the testicle.
Testicular torsion: A twisting of the spermatic cord that cuts off the blood supply to the testicle, causing severe pain, swelling, and nausea. It is a medical emergency that requires prompt treatment.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. It can cause a dull ache or feeling of heaviness in the testicle.

Testicular cancer: Although rare, testicular cancer can cause pain, swelling, or a lump in the testicle. It is essential to rule out this diagnosis, as early detection and treatment can significantly improve outcomes.
Appendicitis: In some cases, appendicitis can cause referred pain to the testicle, especially if the appendix is located near the testicle.
Kidney stone: A kidney stone can cause severe pain that radiates to the testicle, especially if the stone is located in the lower ureter.

Henoch-Schönlein purpura: A rare condition that causes inflammation of the blood vessels, which can lead to testicular pain, swelling, and bruising.
Spermatocele: A rare condition where a cyst forms on the epididymis, causing pain and swelling.
Testicular trauma: A rare cause of testicular pain, usually resulting from a direct blow to the testicle.