Stitches normally come on during hard workouts or races.
If the stitch is on the left, exhale hard when your right foot lands. Continue until pain recedes. Or run with your hands on top of your head and your elbows back while you breathe deeply from your belly. Another remedy is to take your fist and dig it under your ribcage, push the fist in with your other arm and bend your torso over almost to 90 degrees. Run like this for 10 steps.