you might feel pain either from the liver (center-right), spleen (left) or diafragm (center + breathing)
both the liver and the spleen are "sponge" organs, they hold blood (the liver specially). when your body needs an extra blood rush in the stream it drains blood from those organs. if there isn't enough blood in the organs you'll start to feel the side stitch. kinda like a "you're going a lil bit off board there"
the spleen has another usage, it will "clean" the blood of the dead red cells. if there's more dead red cells than the spleen can handle, it will start to accumulate blood. thus the spleen swells a bit and starts to ache
notice that these pains are very similar and they'll happen whenever you're exercizing excessive circulatory stress on those organs
the diafragm pain is the one on the center (kinda) of your chest... its somewhat constant as the side stitches but it will hurt a lot more when you try to do a heavy breathe. its reasons are somewhat unknown but its probably because the diafragm muscle is not used to the speed of contractions and it starts to lack efficiency (kinda like sore legs after walking a lot). another reason might be a lack of blood supply to the diafragm
anyway, its usually just a matter of not being used to it. just get your body to handle it well