Suspect stitch

Walking to school on Monday, Marcus was in a bad mood. He is not a lover of walking for the exercise. His complaints culminated in: "Whenever I get halfway there I always get a big pain here". This sounded plausibly like a stitch, but I thought that was unlikely in a fit 5 year old who has been walking downhill for less than ten minutes. I asked "Do you always get this when you walk this far, or only when you are on your way to school?". There was a telling little pause. "Strangely... only when I am going to school". Ah.