I don't want to scare you, just inform you. I just want you to know some things just in case, but I hope it is just a rash from getting rubbed too much.
It could be herpes from what I've read. Many people don't get bad breakouts, particularly men. That's why so many people aren't diagnosed with it. They get a rash, it goes away and they don't get recurring outbreaks so they forget about it. They only find out after they've transmitted it to someone who gets a much worse outbreak. And they are shocked.
Just so you know what to ask the doctor. Some doctors expect herpes to always be a bad outbreak, so they will dismiss a rash like yours and not do tests. There are two types of tests for herpes. The first is a swab test where they actually rub a swab on a suspected outbreak area, then test that. But if they wait until the tail end of the outbreak, sometimes the virus is no longer present on the skin. The second type of test is a blood test that detects antibodies. (One brand name is Herpeselect.) The antibodies are not present in the blood until a few weeks or months after the first outbreak though. Both tests can frequently give false negatives.
To be on the safe side, don't touch the area. If it is herpes, your body doesn't have antibodies until a few weeks or months after being infected. What this means is you can spread it to other parts of your genitals or other parts of your body. After the antibodies are present, it's much harder to do that. If you do have it you can also get antivirals which are helpful during the initial outbreak because they will reduce the outbreak and make you less likely to spread it to yourself. This will make you have less outbreaks in the future and be less likely to transmit it.
Since it is on the shaft, you would have had to come in contact with someone infected on your shaft. Did you have unprotected sex or oral sex? If so, you can ask that partner if they have it or get cold sores. Many people get genital herpes from someone that gets cold sores and gives them oral.
Since you are worried so much, you can check out this site and the forum there:
Herpes - Sort the Facts from the Fiction
Herpes & Cold Sore Support Forum
Good luck.