Hi,
Are there other signs or symptoms present? This could be tinea versicolor, pityriasis rosea, or a ringworm. Though it is very hard to reach a diagnosis without actual examination of the lesion. Tinea versicolora relatively common skin infection caused by a fungus which presents with patches of discolored skin with sharp borders and fine scales that sometimes coalesce. The patches are often dark reddish-tan in color and skin may appear lighter than surrounding healthy skin and usually associated with hot climates. The most common sites are the back, underarms, upper arms, chest, and neck. Diagnosis is through skin scraping and subsequent examination under a microscope. Treatment consists of over the counter topical anti-fungal medicines. This link may be of help: http://en.wikipedia.org/wiki/Tinea_versicolor
Take care and keep us posted.
Try Butt balm serious that's the name.