What To Do For The Dark Spot Under The Lips?

Hello, I am a 20 years old female weighing 110 lbs, and 5'0. Just last October my doctor concluded from my blood test that my hemoglobin levels were really low, so he put me on iron injections for a month or so and afterwards iron supplement pills. Since December last year I have had this unusual looking black "spot" below my lips (kinda looks like what men would keep as a Go-Tee). I went to the doctor again (about three months ago) and he gave me a cream to apply twice daily but there is no difference in the spot. I read somewhere that these could be permanent because of the injection given in the muscle of the buttock (which I got). What does this look to you, and what do you recommend I do?