that could be any number of things:
-if it is the right could be that your left leg is lagging or vice-versa
-exhale when your left foot strikes the ground, if you do on your right your diaphram puts pressure on your liver.....they told us that at basic, there's asomething over there
-crap from not drinking enough fluid the night before a run esp. a long run
-could be your out of shape
-the list goes on pick one