Periodic purple blotches on arms?

Purpura. When you suffer from a condition known as purpura, blood vessels anywhere within your body actually begin to leak, causing purplish spots to develop along your skin. Causes can be low platelets, sun damage, medications, immunologic, infectious, and systemic diseases. See your local doctor for help in discovering a cause.
Purple blotches. Usual causes for purple blotches is a condition called purpura. It represents bleeding under/in the skin and may be caused thinner blood or thinner skin.