Spots on arms, pink and round, and on legs they are red and flat and round. not allergic to anything, no pets, biopsy was neg. what's causing them?

Pityriasis rosea: Pityriasis rosea is a perplexing rash that usually begins with a single oval patch ("Herald Patch"), and then spreads symmetrically on the chest, trunk, arms and legs. The location may vary somewhat. No known cause. Not contagious. No known treatment. Good news is that it will go away, although it's been known to linger as long as several years. If it doesn't itch, you're lucky.