Most common swelling in scrotal sac apart from testis is a hernial sac, outpouching of abdominal contents. Other common swellings may arise from water collection around the testis hydrocele), testicular head (edididymal cyst) or other contents like connecting cord (hydrocele of the cord, lipoma). It seems that your swelling keeps on disappearing and coming back with constipation, in other words abdominal straining. Such swelling is more likely to be a hernia. You must see an urologist who would examine you and have appropriate investigations to diagnose it correctly.