I Am A Very Active 60 Yr Old Man In

I am a very active 60 yr old man in great shape, and currently in a pretty warm tropical environment with about 80% humity. Anyway my question is that late yesterday I noticed large red rash looking marks on my lower shins that don’t hurt or itch, they are still present on lower legs and are about 3-4 inches in diameter, do you know what these marks are?