Hello,
For such kind of hyperpigmentation below the chest, creams containing alpha hydroxyl acids and retin-A can be used. If the symptoms persist then Laser therapy and dermabrasion, alone or in combination can also be tried.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I forgot to include a pic:
http://img138.imageshack.us/img138/6848/p1270062.jpg