What could cause skin discoloration all over my back?

Tinea Versicolor: Tinea versicolor is a common yeast infection of the skin that often affects adolescents and young adults. The term versicolor refers to the fact that it causes the affected skin to change color and become either lighter or darker than surrounding skin. Treatment can be with prescription Nizoral (ketoconazole) tablets although the condition will return as there is no cure.

Many possibilities: There are many things that may cause discoloration of the skin. Several types of inflammatory conditions of the skin, sun exposure, allergic reactions, bacterial, viral or fungal infections, insect bites, friction, and cancer can all cause discoloration of the skin. Have it evaluated by a professional and get the appropriate treatment.