if you have had recent unprotected sex or oral you know what to do to get tested.
If this is not related to sexual activity there are a number of things it could be, these can be lumped together under the term skin imperfections, and of course causes can range widely, from amount of pressure present in the area, such as your penis being erect for sometime and too much pressure being applied. people who use penis pumps and developers can get this kind of thing, or it simply may be an imperfection that has grown within the skin, such as a general wart you can get anywhere on the body.
I start by treating things like this with undiluted liquid antiseptic and see how they react, if it begins to heal use the liquid antiseptic until it is completely healed. If it does not get better and you are suspicious of what it is see a doctor …