Well, the rash on the penis could be of various reasons, simple to complicated. Simple for the fact that if you have worn very tight undergarment, or the undergarment is not dried well in the winter season or the rainy season. It could be a fungal infection that could be one of the reason, simple reason. Second reason could be an injury. Other than these two it could be many other serious conditions , well , it could be sexually transmitted diseases, for which you have to get a bacterial test done to rule out various other conditions it could be syphilis, it could be gonorrhea, it could be a simplex virus, also herpes simplex virus what you call or any other condition, which has to be ascertained by examinination of that area by the physician and so now other tests. It could also be because of a simple fungal infection , please do contact your doctor to find out that exactly and start treatment. It is very important that you get treated. If at all you are married or you have a girlfriend, make sure that you get treated fast, and it is very important that you do not have any sexual intercourse with your partner till you get cured.