Lumps on testicle may be due to many causes and testicular cancer is one of the rare but important causes. Most of the lumps are harmless. Although any abnormal lump observed on self examination must be brought to doctor's notice.
Lump in testicular cancer is usually painless. Other signs to be noticed are if the lump is growing, any associated abdominal pain or any lymph node swellings in the groin (inguinal) region. An epididymal cyst due to collection of fluid in the tubule of testes called epididymis can present as painless lump and is called spermatocele when it is filled with sperms. They are harmless and do not require any treatment.
Varicocele due to enlarged vein of the pampiniform plexus of veins in scrotum can present as palpable swelling. Varicocele may grow slowly and sometimes effect sperm production. It may cause a mild dragging type of feeling or pain. Small varicoceles are usually left alone and surgery is required in cases of large and symptomatic varicoceles.
Excessive fluid accumulation due to secretions from covering of testes called tunica vaginalis or due to blockage of inguinal lymph channels draining scrotum can present as swelling in testes. This is called hydrocele and is often painless. Hydrocele can occur due to trauma, infection or cancer and should be examined. Surgery may be required in enlarged hydroceles. Scrotal skin has sebaceous glands which may get infected or blocked and present as small lumps in the skin. These sebaceous cysts may get painful and pus filled at times.
I suggest you to have a doctor consult for testicular examination to evaluate the lump. Although rare, any abnormal lump on testicles must be checked for testicular cancer.
Hope this helps. Thank you.