The reason you get a red face after sports is due to the effect of the effort of working out. As you do sports, your muscles toil harder, so they use up oxygen more rapidly. The heart will then thrust blood quicker to keep up with the high demand of oxygenated blood. This causes the blood streams in your face to expand for increased flow of blood thus, causing your face to appear red. See http://www.livestrong.com/article/294929-why-does-my-face-get-so-red-when-i-exercise/.