It maybe postinflammatory hypopigmentation.It can happen if you have a habit of lip smacking or sometimes on its own....I would suggest you to use melbid lotion as It gives good response and is easily available...you can apply the lotion in the night and let it be there for over night. in morning sit it front of a table lamp or sunlight with light falling on your face for few minutes..if no improvements consult dermatologist they will do the needful.