exerpt from this article: http://www.breathing.com/articles/diaphragm-stitch.htm

stitch is a form of cramp, normally experienced in the diaphragm near the right side of the rib cage. It usually indicates that you are pushing yourself too hard.

In some cases it can happen quickly, striking without warning and causing excruciating pain. In other cases it can come on gradually, starting as a slightly uncomfortable tightness or tense feeling. Most people try to work their way through this type of discomfort taking it as a normal reaction to the exercise they are doing. But as they continue the pain increases - to the point where the cramp sets in, causing the same excruciating pain as experienced in the sudden attack.

This type of stitch is felt on the right hand side of the chest as a direct result of a cramp in the diaphragm. If you experience a cramp on the left hand side of the chest this is normally caused by eating too close to your workout period.