When you exercise your skin gets hot.
When your skin gets hot, your pores open to let the heat/sweat out.
When the pores open, whatever is on the surface of your skin can get in the pores.
When there is foreign matter (makeup, dust, whatever) in your pores, they become inflamed and try to clear themselves. Thus, acne.
"If hunger is not the problem, food is not the answer." ~Silversun (and others!)
"GWF powers, ACTIVATE!"~dangerousfish
"How does one become a butterfly?" she asked pensively.
"You must want to fly so much that you are willing to give up being a caterpillar."~ Trina Paulus, Hope for the Flowers