Penile Lymphocele. here is some info from the web:
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This is a hard swelling that suddenly appears on the shaft of the penis after sex or masturbation. It happens when the lymph channels in your penis are temporarily blocked. Lymph is a clear fluid that forms part of the body’s immune system.
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The only cure it to let it heal itself, this can take a matter of days or even months - best to avoid sex and masturbation, if you use any form of penile enlargement avoid for 5 to 8 months even if the Lymphocele heals