It sounds like it could just be irregular pigmentation of the skin. Tight bras would not cause the skin to change color unless it's an area that it is constantly rubbing up against.. And even then it would be red or have dry skin, not what you are describing. Maybe see a dermatologist to see if they have a way of minimizing the discoloration.

Maybe it's a birth mark???

Hi. I had what sounds like the same thing and was embarassed to ask a doctor, but I did! He said it was a fungus on the skin and gave me some topical cream to put on it and "presto", it did disappear. So, ask your doc!
Good luck.