J.I.
I did not have a very noticable "linea negra" when pregnant with my son, but, I did have Chloasma, "the mask of pregnancy". I had dark freckled patches on my forehead, and the sides of my breasts. The same as you, I thought they would go away quickly, but, it took about 3-4 months for them to fade, and eventually disappear.
Don't worry, it will go away, with time!!