Do you feel that it is something to be truly be concerned about? Is it causing any major issues? I just am asking because some people just have discoloration at times on their skin. I know my little girl has white patches on her neck...and it is perfectly normal. She doesnt like them because they make her different but I have explained different isnt always a bad thing.
You can always get a second opinion to see if another doctor maybe able to clue you in more on what is wrong. Did anything happen 5 yrs ago that could have caused this? Could you have burnt your lip or injured it in some manner? If so it could be a scar. Either way if it is not causing you any harm, while it affect your outer appearance you shouldn't let it stress you out. I am sorry I could be of more help but I hope this was some help.