Pain in the side of the diaphragm.
Running downhill increases the forces exerted on the entire body with each foot strike and may induce side stitches, especially if you are nervous (for whatever reason) about running downhill to begin with! Try walking or slowly jogging down any steep hills until you have mastered deep breathing techniques.
If you tend to suffer from side stitches, try to avoid eating within 1 hour of running. Water or gatorade is fine within an hour -- liquid empties from the stomach faster than solids and shouldn't cause any problems. Running in extreme cold temperatures may induce side stitches, as it's less comfortable to take in deep lungs full of frigid air until you're thoroughly warmed up. Try doing some sort of warm up indoors first, before hitting the icy city streets.
A "side stitch" is a sharp, intense pain under the lower edge of the ribcage caused by a muscle spasm of the diaphragm. Such pain can occur during vigorous exercise, such as running, and seems to occur more commonly in novice exercisers who have not yet established proper pacing and who tend to breathe more quickly and shallow. However, about 30% of all runners will experience stitches at some point. What exactly causes them? On inhalation, we take air into the lungs, pressing the diaphragm downward. When we exhale, the diaphragm moves up. If the body has some trapped air/gas below the diaphragm, if we've eaten too close to exercise, or if we start exercising too vigorously, the diaphragm may cramp, causing pain under the rib cage on the right side.
As with any muscle cramp, the best immediate treatment is to try to stretch the cramping muscle as much as possible. How do you get to the diaphragm on the inside of your body?, Try altering your breathing pattern. Take a deep breath in as quickly as you can, to force the diaphragm down. Hold the breath for a couple of seconds and then forcibly exhale through pursed lips to restrict the outward air flow. You may also find that bending forward can help you expel as much air as possible. I have actually found that stretching up as tall as I could, even to the point of extending arms up over head, then alternating crouch-tall and tightening/flexing the abs, helped as well.
You may even have to stop and walk briskly for a few seconds while concentrating on deep breathing. Continue running after the stitch goes away. If you get a cramp in the middle of a race, you might want to try mixing up your rhythmic breathing/ striding pattern. If you always exhale when your right foot strikes the ground, try exhaling with the left foot strike. The organs attached to the diaphragm on the left side of the body aren't quite as big as those on the right side, hence there is less strain on the diaphragm. Another technique that may work for some is peaceful visualization--if you are feeling stressed from the day or race, try imagining you are elsewhere, and take deep calming breaths as you run.