Is the swelling both sides of the abdomen? From what I can see of the photo, it looks like a little football. Like I said, you could try peas but I don't think it is constipation, it could be though. If the fish is eating, you could try soaking some kind of internal parasitic treatment just in case it is something like that. Or you could try an internal bacteria medication. This is often what causes swelling in the abdomen. However, it could also be problems with the internal organs, sometimes you get fluid in the abdominal cavity when this happens. If you're able to gently squeeze the abdomen, it would be squishy, that could be fluid. Unfortunately, there is no cure for things like this.
I should avoid giving your fish feeders, unless you breed them yourself. There is always the possibility that you can pass things on through this type of food