Side stitch

Gbex says :Why do you sometimes get that pain in the side when you're exercising? Nobody knows for sure, nor do they know for certain what it really is.It usually affects those who are just beginning a conditioning program rather than those who are already in good shape. One theory suggests it is due to insufficient oxygen getting to the diaphragm.

Whatever the case, the pain will go away if you slow down a bit and take a few deep breathe.