I've noticed my chicks bottoms all seem to heave, it's just part of their anatomy when they breathe. But if it looks much different than the others it could be something. 
 
If you haven't seen it poop in awhile you might want to try soaking its bottom in warm water. You could take it out for awhile first and see if it poops at all in an hour (it will not be happy to be away from its friends). I've read somewhere chicks poop up to every fifteen minutes, so it should be pooping within an hour.  If it doesn't poop maybe try a short soak?