Hello,
It can be due to tinea versicolor or idiopathic guttate hypomelanosis. Idiopathic guttate hypomelanosis is characterized by flat white spots found on the shins and forearms. It may also arise on other sun exposed areas including the face, neck and shoulders. Usually nothing is to be done except sun protection. However if it is tinea versicolor then topical antifungals like Selsun or Nizoral shampoo help.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Most likely photodermatosis, but then tinea versicolor a distinct possiblity.