I understand your concern.
Thank you for uploading the image for my better understanding and also the information that it started probably after masturbation - you are right indeed.
This looks like Lymphocele of the penile shaft.
This is a 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.
The swelling should soon subside and won't cause any permanent problems.
It does not require any treatment as such and will settle down in few months time.
I am happy that my interaction was helpful for you and put your mind at ease.
In future you can opt me as your favorite medical expert so that you can ask me a question directly. I will be happy to help anytime.
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Kind regards ***** ***** Sumanth Amperayani