Likely, runner's stitches. They're awful and can stop you dead in your tracks. From what I've read the causes are largely unknown. There's a few theories, however.
You can read more about it here: en.wikipedia.org/wiki/Side_stitch
I find if I eat something, especially fruit before exercising I get them but sometimes still get them when exercising in a fasted state.
You can either take a break for a bit from exercising and stop and massage the area but a lot of times I find it returns quickly again when you start back up. I find a good abdominal stretch helps with a 5-10 minute break.
ETA: I just realized you said "upper chest". Runner's stitches occur below the ribcage. Sorry, I jumped the gun there.
This article may be more helpful: www.livestrong.com/article/423926-sharp-ch
Edited by: JENNILACEY at: 9/27/2013 (14:27)
Take your focus off the Marshmallow. www.leangains.com/2010/01/marshmallo
"Toning" is marketing muscles to women who are afraid if they pick up a barbell, they'll leave the gym looking like She-Hulk. It doesn't happen, what does happen is you get results. Lifting Barbie weights does nothing but waste time.
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