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What is your age might I ask. You probably have a Hernia
. The history you gave is quite typical, of the swelling disappearing after you lie down. Hernia is a mass of usually intestine which comes from the abdomen into the scrotum
due to weak abdominal walls. It would also come down in situations when you lift heavy weights, laugh, cough
. You will need to visit your Doctor to confirm what the swelling is and get appropriate treatment. For hernia, corrective repair surgery is usually indicated to push back the bowel inside and repair the weak part of the abdominal wall.