How to you get cold sores on your bottom?

Possible herpes?: One possibility is herpes, which is a viral infection that lives in a nerve tract once it enters the body and may recur from time to time. Other possibilities may relate to pressure on the area or other dermatologic causes. Less obvious cause could even be gluten intolerance. All conjecture until a dermatologist actually sees it and hears more about the history. Don't try to treat it yourself.