Dear Welcome to Healthcaremagic! I have gone through your description. I find that the symptoms described by you match closest to a condition called Lichen Planus Pigmentosus. In simple terms, it s ia disease of body s defensive system in which there are purplish discolored mottled patches seen on different parts of skin but more so on the exposed areas. Just as you said, these lesions are flat and may not be related to sun exposure. I would advise my patients of this kind of problem to apply Hydrocortisone cream and use some Anti allergic like Cetirizine. it helps in almost all cases. the disease tends to get better with time and hopefully you too will be well soon. Best wishes