I have this too, I've had it since my late teens (I'm 32 now). I've always hated it with a passion, b/c when the rest of my skin looks good, and I want to go out w/o foundation for a change, I can't b/c of my upper lip. AAArrgh! I don't know what caused it for me, since I was so young then, maybe hormones of puberty? But I went thru puberty early, so I don't know why in my late teens I got the discoloration. I know for some women, pregnancy causes it and it is called melasma. Also it is more common in women of color. I have never tried any of the lightening treatments b/c I am afraid of a "halo" effect, where the discoloration does lighten, but so does a border of normal skin around the discoloration. THen you end up with another problem. Also, I love the sun and I let myself get tan, which you are not supposed to do with lightening products. *sigh* so in other words, I just live with it and cover with MU.
If you are not a woman of color, a lightening product may be the answer.