Hi Anna, I'm not being thick and dont like to take a stab at things that are not clear, but can you please expand you question a bit more and a little bit more detail.
Hi and welcome, im not a doctor but have some experience. If its on the head, then it could be an allergic reaction to medication. The end of the penis is where a lot of reactions to meds end up. Either high blood pressure chlestrol medication can do this Has something to do with gravity and blood supply. Have it checked out with the doctor maybe a dermatoligist. They may just put a cream on it called cloderm or if take a small biopsey.
good luck.