Hi,
A differential for your case is tinea versicolor .This fungal infection usually presents wth white patches in the back,neck and arms.Tinea versicolor may be easily treated with antifungal medications however, the white skin patches may take quite sometime to match the normal skin color.
Another differential is pityriasis alba.However,the white patches of pityriasis alba are multiple and appear to be more confluent.In your case, the more common tinea versicolor may be the underlying cause. I suggest you seek consult with your primary physician so that the right medications be prescribed.
Avoid sharing towels and soaps since tinea versicolor as a fungal infection may easily be spread.
I've had the same thing! Twice, actually. Does it last long or just disappear after a few days?
The doctor told me it was a virus, and to let it run its course because there was nothing he could give me to make it any better. I suggest turtle necks, if its the same thing.