Well technically: A confused state of mind acts upon the sympathetic nervous system. From there, the vasodilators are stimulated which causes the peripheral capillaries to expand. As a result, more blood flows to the surface of the face and neck, resulting in the reddening of the face and neck.

HAHA! Im kidding. It is apparently a bodily defense mechanism. When we get nervous our heart goes crazy and our adrenals go nuts, so to compensate, the blood flows to our brain, thus making our face appear red or pink.

There you go. Brought to you by my brain's folder of useless information.