I think a stitch is a cramp of the diaphragm. I have to be careful what I eat before running as too full a stomach will cause them due to all the food bouncing around. I used to get them cycling a couple of years ago when I first started mountain biking. I think it was due to my "breathing muscles" (certainly around my ribcage) being put under a lot of strain with the inhalation effort of going up steep offroad hills. I still breathe hard on those efforts but I think my body is used to it now and can cope. Once I had a stitch I really had to calm my breathing down to try to get rid of it.