here's part of an answer from another place:

. Cancerous lumps in the testes are generally not tender when they are touched. Relatively rare, testicular cancer usually occurs in men between the ages of fifteen and twenty-five years, especially if they have an undescended testicle.

Other causes of swelling and/or pain in the testes can include:

* hydrocele, a collection of fluid in the scrotum
* epididymal cyst, a fluid-filled swelling of the epididymis (the structure behind the testicle where sperm mature) which may also cause fever and discharge from the %#@&#!
* varicocele, varicose veins in the scrotum, which is described as feeling like "a bag of worms"
* bacterial infections

Torsion of the testis, another possible explanation, occurs when a testicle rotates, obstructing the tube sperm travels through and cutting off the blood supply. This most commonly occurs around puberty and causes acute severe pain and swelling of the testes. Pain in your testicle is most likely not a hernia (a protrusion of an organ or tissue through a weak area in the muscle or other tissue that normally constrains it, which commonly occurs in your abdominal area).

If during sex or masturbation you are engaging in activities which involve twisting, pulling, or squeezing of your testicles, this may be a cause of pain. Otherwise, average masturbatory or sexual techniques (even many times per day!) do not cause your testicles to swell.