This is due to vasodilation.

"Vasodilation occurs in superficial blood vessels of warm-blooded animals when their ambient environment is hot; this process diverts the flow of heated blood to the skin of the animal, where heat can be more easily released to the environment. The opposite physiological process is vasoconstriction. These processes are naturally modulated by local paracrine agents from endothelial cells (e.g., nitric oxide, bradykinin, potassium ions, and adenosine), and by the autonomic nervous system and the adrenal glands, both of which secrete catecholamines, such as norepinephrine and epinephrine, respectively.[6][7]"

It's the opposite of vasoconstriction, which occurs when you're cold and your extremities have less blood flow to prioritize the warmth of your core organs.