About a Neon Tetra behaviour.?

I have a small tank with 12 Neon Tetras and 6 Rummy Nosed Tetras. One of the Neons has started behaving strangely. Its almost like it is constantly trying to swim down but keeps floating back up (looks like its bouncing) but never breaks the surface. Its also quite a bit fatter than the others. Not sure if this is due to possibly spawning or could it have swallowed some air. The water is good and the fish are not over fed. Thanks for any help.