Since you have natural dark lips, so there is no cream or ointment as such which can lighten the lips except for laser therapy and cosmetic surgery which has shown some effective results. I suggest you to consult a dermatologist regarding this.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.