It's a stitch. We think it's caused by spasm of the diaphragmatic muscles. More often on the right vs. left. we really don't know what causes them but improper breathing, dehydration, improper warm up etc. may all contribute.
see link http://my.webmd.com/content/article/...tselectedguid={5FE84E90-BC77-4056-A91C-9531713CA348}