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A soft lump in the scrotum, especially if you can get your fingers between it and the testicle, is most likely to be a hydrocele. This is a collection of sterile fluid in the cord attached to the testicle, which is invariably harmless. Left untreated, a hydrocele can grow to a large size and cause discomfort, so they are usually best drained early.
On the other hand, Epididymal cysts are fluid-filled cysts that come from the epididymis, and are very common. Often discovered when about the size of a pea, they feel slightly separate from the top of the testis. These lumps can slowly increase in size and can feel like a third testis. Epididymal cysts can happen at any age and should be left alone when still small, but can be removed with surgery if they become larger.
These are the two common non-cancerous condition which causes pea sized swelling on testicular sac. Cancer is an uncommon condition, so you need not panic about it right now. I would advise you to consult with a male gynaecologist and examine the swelling, if necessary an ultrasound of the testicle with the swelling might be asked for by the doctor to diagnose the condition. Take an appointment from a gynaecologist as early possible. I would advise you not to panic about cancer right now.
Hope this helps. feel free to ask if you need further help,