Have flat grey circular marks on for 2 weeks. Fade and get darker thruout day. No pain or blisters.?

Biggest one is about the diameter of pencil eraser. Others are half that. Bordered by white line which also fades and darkens periodically. No itching or burning. Any ideas? They just appeared out of nowhere....I've been monogamous for 7 years. Could it be STD or fungus?