Is something wrong with my neon tetra?

I've been watching this fish for a while and trying to collect clues, but "he" has me baffled. For the most part the fish acts normal and healthy. The color is the same as my other neons, it still hangs out with the other fish most of the time, and I don't notice any external parasites or damage. However, instead of having the traditional elongated oval shape of a neon, it's "stomach" goes straight across and is even a little concave. My first worry was that this signified neon tetra disease, but I don't notice any other symptoms at all. I'm worried about "him," but can't for the life of me figure out whether he's sick, or if he just doesn't eat a whole lot, or what. (I've seen him eat at least on occasion, ) I got the fish a few months ago, and he was shaped normally. This seems to be something new over the last month or so. I had a high ammonia level for a couple of days and have been doing partial water changes every day or two and that has appeared to clear up.