Disclaimer: I am not diagnosing, which would require a careful history and physical evaluation. You should be seen by a health care provider for evaluation and treatment. Diabetes and other conditions can cause skin discolorations.
Based on this one photograph, and without any mention by you of a fever, other rashes or open sores, significant health concerns, etc, I would say it appears to be either tinea versicolor or a monilial (yeast) rash. Both are fungal, and thrive in areas of the body that tend to be moist and dark. The area under the breasts fits the profile because you can have sweat accumulate there. In fact, the back, chest, and upper arms are common areas for tinea versicolor.
Initial treatment for tinea versicolor is Selsun Blue shampoo applied to the entire upper trunk and head (avoid entire face) daily for 1-2 weeks. Leave the shampoo on the areas for 10 minutes, then rinse well. Some people get contact dermatitis from Selsun, especially if used longer than the recommended time. If you do get a breakout from the Selsun, stop using it and see your practitioner.
Yeast (fungal) infections can be treated with an OTC antifungal agent (for instance, Monistat Derm). It is applied twice daily for 1-2 weeks.
Your health practitioner may prescribe a prescription medication, Nizoral, if tinea versicolor is diagnosed. Make sure you have healthy liver function prior to taking this med. To be most effective, take it with a can of cola beverage (the acid helps) and then exercise to induce sweating about an hour after you take the pill. That helps bring the medicine to the skin, where it is most needed.
Source(s):
Hooper and Goldman, Primary Dermatologic Care (1999). Mosby.


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