NEON TETRA HELP PLEASE!?

My neon tetra has a really big belly, **NOT PREGNANT** ive seen pictures of pregnant tetra, and its different looking. He is not constipated either. he's been like this since i got him but im worried because my other neon tetra had the same problem and his now really sick. My tetra swims fine, stays arond the other tetra all the time, im just worried for the long run.