james and zaohu have it right, it is your breathing stretching the ligamentum teres of your liver when you exhale while landing with your right foot (exhalation is caused by elevation of the diaphragm which thus elevates the liver, stretching said ligament, combined with stretch of your right leg leading to the side stitch).
BUT, I must disagree with zaohu about heel striking. Dont do it. Take off your shoes and go for a run, you will find out very quickly that you will naturally run on the balls of your feet. People are built to run and modern tennis shoes are the only reason people can heel strike for long distances.
But, you can forget that, just think of it biomechanically. If your heel is landing in front of you, then you have no way to propel yourself forward. Instead you must wait for your forward momentum to carry your hip over your heel before you can propel yourself forward. Not to mention much of your forward momentum is absorbed by your bones from the impact. By running on the balls of your feet, you store that forward momentum in the muscles and tendons of your feet and lower legs, returning much of it to your forward propulsion. What is more, the second your foot lands (basically under your pelvis) you are pushing off again. Add to it the growing evidence that connects heel striking to injuries and lower leg weakness, and there really is no reason to keep heel striking besides stubborness.