Dark spot on lower lip

[ryan4646]

I developed this dark brownish tinted spot on my lower lip a couple months ago. No idea what is it. I went to see a dermatologist about it and he didn't really offer anything that could help me. He gave me some tri luma which is lightening cream but that hasn't help. It seems like it gets more/less visible depending on what kind of lighting i'm in (sunlight, brightly lit room, etc). I really want it gone but not sure what more I can do. Has anyone else experienced something like this or can offer any advice to get rid of it? Here are some pics:

https://postimg.org/gallery/ttutj2ea/8076f56d/

Thanks,