If you are really skinny, it is possible you are seeing the pulsations of each heart beat against the chest wall.
http://en.wikipedia.org/wiki/Apex_beat
It is, though, often seen to the left side of the chest, since this is the side where the bottom of the heart lies.
If you're talking about lower down in the abdomen, you may be seeing the normal abdominal aorta pulsating, which again can be seen in skinny people.
Some people may have an "aneruysm" of this aorta:
http://www.christianacare.org/images/SIR_AAA_aneurysm_2.jpg
But this is usually seen in older people, starting around the age of 50 and up.
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M.D. - Neurological Surgery