What Causes White Blotches On Arm?

i have white blotches on my arm..they came after been out in the sun. i have them over a month now and there not fading. i seem to be getting more and there starting to appear on my other arm mostly on my shoulder.went to the docs bout 3 weeks ago and he say it just virus and give me nothing for it. he said it go away in two weeks but it hasnt..can anyone tell me what it is?????