GREETINGS! Your symptoms are not clear or suggestive of any disorder definitively; this could probably be milaria - due to the hot weather. Kindly send in pictures of the rash when you have it next - on follow up messaging - or you could see a qualified dermatologist near you. Skin around joints is usually a little darker - if it bothers you - you could try using a glycolic acid based cream for lightening. Hope this helps.