When you run, you use more oxygen than you do when you're walking or resting. If you aren't able to compensate by pumping your blood faster and breathing more deeply, your tissues may start to feel oxygen deprived.

Fortunately, you aren't out of options yet. Your spleen, which is in your abdomen, stores a small volume of blood which it can release into circulation on occassions like this. By increasing your circulating blood volume, you increase your "oxygen carrying capacity"--that is, you can carry more oxygen in your blood because there is more of it.

Cool, but what does that have to do with a stitch in my side you may ask. Well, in order to get that extra blood out and circulating, your spleen contracts. This squeezes the blood out--but it also feels uncomfortable, kind of like a muscle cramp. Or a stitch in your side.

- Answer provided by Ask Dr. Universe