The possible causes of swelling and pain above the testicles on one side of the penis as soon as it is engorged with blood are acute orchitis, varicocele, trauma and so on. 1. Acute orchitis: this disease is usually accompanied by a variety of acute infectious diseases, such as acute orchitis, the clinical manifestations of testicular swelling and pain, scrotal redness, high fever and so on. 2. Varicose veins: this condition refers to the diseased people whose spermatic veins are varicose like earthworms, presenting a mass or nodular shape, with triggering factors such as standing for a long time, obstructing venous reflux and causing persistent venous pressure elevation disease, etc. Clinical manifestations include the enlargement of the scrotum on the affected side accompanied by a feeling of swelling and hidden pain. 3. Trauma: When the testicles are hit by external force during exercise, it may cause swelling and pain, etc. Penile congestion will pull on the scrotal tissues and cause related pain symptoms. If the pain symptoms persist, it is recommended to go to the hospital for consultation and targeted treatment.