There isn't sufficient information to reach a specific diagnosis. The lesion would require examination, I would advise you to visit a dermatologist so that you can be examined and undergo the necessary investigations such as iron, vitamin B12, CBC, etc. to find the underlying cause of the problem. If you have a habit of smoking this can also cause the discoloration so kindly stop it at the earliest. If the problem persists or worsens visit the ER at the earliest.

This is limited information and provide minimum about your skin condition which may not help in planning a treatment.  A thorough history, clinical examination and appropriate laboratory tests will be needed to solve the mystery. Consider seeing a Dermatologist for proper diagnosis and treatment plan. Use a lip balm with sunblock. Medical condition that can cause darkening of lips should be ruled out by meeting a Dermatologist.