That's an excellent question--one that many runners ask--and yet I can find no definitive explanation.

Here's an article at Runner's World that suggests it might be a sign of "exercise-induced anaphylaxis":

http://runningtimes.com/Article.aspx?ArticleID=7006

except:

The chemistry of exercise-induced anaphylaxis may be the same as any other allergy, says the Food Allergy and Anaphylaxis Network (FAAN; www.foodallergy.org): The victim’s immune system senses erroneously that a harmless substance (or in this case, exercise, or the combination of food and exercise) is dangerous. The physical manifestations may begin with a tingling sensation, itching, or metallic taste in the mouth.

Other symptoms can include hives, a sensation of warmth, asthma symptoms, swelling of the mouth and throat area, difficulty breathing, vomiting, diarrhea, cramping, a drop in blood pressure, and loss of consciousness. The disorder generally isn’t considered life-threatening, although doctors “don’t yet have enough research data to know if this type of anaphylaxis is as serious as anaphylaxis to, say, [a] bee sting or peanuts,” says Carlos Camargo, MD, associate professor of medicine and epidemiology at Harvard Medical School. In fact, “we’ve learned precious little about exercise-induced anaphylaxis—and about anaphylaxis in general—in the past 20 years,” says David Golden, MD, research associate professor of medicine at Johns Hopkins University and fellow in the American Academy of Allergy, Asthma and Immunology. “We have many questions, and few answers.”