Thanks for providing additional information.
As per your description,this may just be a bruise from the vigorous masturbation.
Other cause of brown spot is just suggestive of dry skin.
This is a common condition affecting many men and no treatment is required since the condition is harmless.
Things which can help are - avoid vigorous washing with a soap, instead use a mild soap and wash off immediately after lathering on; and use a moisturizer like cetaphil cream immediately after you shower.
While having sex and masturbating use a lubricant such as KY jelly.
All of these things will help prevent drying out of the penile skin and thereby decrease the appearance of this brown spot.
If you 'll develop itching, pain then fungal infection should be ruled out.
Otherwise most likely your symptoms 'll resolve in a week.
If your symptoms get worse or persist more than a week then consult your family doctor.
Hope this helps.
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