It can be idiopathic guttate hypomelanosis or vitiligo. Idiopathic guttate hypomelanosis is 2 to 5-mm flat white spots found on the shins and forearms. It can be due to sun damage or ageing. Treatment options include protection from the sun, cryotherapy, dermabrasion or microdermabrasion, tretinoin cream or pinch grafts of normal pigmented skin.
Vitiligo is a pigmentation disorder in which melanocytes (the cells that make pigment) in the skin are destroyed. As a result, white patches appear on the skin in different parts of the body. Diagnosis can be confirmed by external examination, blood tests to check skin cell count for melanocytes and biopsy of the skin patches.
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 dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.