Hi,
Are there other symjptoms present? A painful testicle mass in one or both testicles may be due to epididymitis, infection of the scrotal sac, orchitis (testicular infection), testicular torsion, spermatocele (a cyst-like mass within the scrotum that contains fluid and dead sperm cells) or varicocele (a varicose vein along the spermatic cord). A testicular lump that does not hurt may be a sign of cancer. Most cases of testicular cancer occur when men are between ages 15 and 40, although it can happen at older or younger ages. A complete physical examination is important as well as some diagnostic tests to help with the diagnosis.
Take care and regards.
Common causes - Hydrocele, Epididymitis.
Rare causes - Testicular malignancies, Blastomycosis, Burkitt lymphoma.
All need investigation and treatment by a spl asap.