Itching certainly can cause anxiety and anxiety increases itching so this problem can become a vicious cycle. I have a brownish darkening of the skin, in a really unattractive mustache shape, on my upper lip. I think it is caused by tweezing and sun exposure. I have never been pregnant, and I've never taken birth-control pills. Although I stopped tweezing, this mustache won't go away.
Skin color is determined by the melanin pigment in the skin cells.
Occasionally something will prompt an overproduction of melanin and an area of the skin becomes darker. In your case, a point to consider is the trauma produced by tweezing. When this occurs over a long period of time, bleeding occurs at the base of the hair follicles and may produce the color change you describe. After bleeding occurs, some of the residue that is not absorbed becomes discolored and affects the skin color. You might talk to a dermatologist about remedies.