Jade:
Most likely, you are describing tinea versicolor, a harless (and not catchy) superficial fungus infection many young people get on their upper back and arms. Try applying Selsun Blue shampoo and leaving it on overnight 7 days in a row before washing it off in the mornings. If that doesn't work, get in touch with your feminie side and see a doctor, so you can find out the exact nature of what you've got, and get rid of it.
best.
dr. R