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Hi. Welcome to JustAnswer. My name is Uzair.I am reviewing your post and will get back to your reply momentarily.
Since it started after harsh masturbation it can be just a skin irritation and inflammation due to friction rub. I would recommend you to apply polysporin ointment over the area and if it does not improve in a week time then get an STD screening. Nothing much to worry for now it is less likely to be STD.
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