A lip ulceration
"cold sores" are a laymen's term for herpes labialis.
This is a viral infection caused by the herpes simplex virus
type 1. An alternative name for the condition is a "fever
blister". It is often painful, but not always, may become crusty and is contagious. The condition usually resolves within two weeks but may reside as a life-long virus. Prompt topical and oral treatment may prevent eruption.