The possible causes of pain in one testicle are: 1. Unilateral epididymitis, in which the testicle is swollen and painful, the testicle is enlarged and the scrotum is red and swollen, and the testicle is sagging. 2. Painful sensation in the testicle. If the scrotum increases in size and the epididymis is obviously swollen and involves the testicles when touched, a mild swelling of the testicles may occur. The diagnosis should be further confirmed by ultrasound examination of the scrotum and routine blood tests.3. Testicular torsion, which is a urological emergency, can present with acute pain in the testicle, which worsens progressively over time. The position of the testicle can be moved upward and the testicle cannot be easily pushed. Ultrasound examination of the testicle is needed to confirm the diagnosis and immediate repositioning surgery is required.