skin irritation?

[Moose]

Whenever its usually really hot or warm some parts of my skin( legs, arms, neck) appear to have red blotchy spots. not really a rash. no broken skin, nor raised skin.
simply really warm to touch and goes away when cooled down.
its extremely noticable and quite embarrassing especially during the summer.
what could it be ??
its not little spots nor somewhat of a bumpy rash.
just weird blotchy red marks.
kinda looks as if you had your knee resting against your leg and it leaves a mark.

ive researched everywhere and cant find an answer.
anyone have any ideas??