Blister?

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I thought it was just a little pimple at first (i get pimples on legs, arms etc.) but I looked at the thing today and its swollen and become purple. It's about a quarter of an inch in diamenter, and raises off the skin on the shaft of my penis about the same measurement, maybe more. Help?

It might be a swollen vein valve, or simply a blood blister, but I think something like that should be shown to a doctor.

thanks.