Green severum showing a big stomach

It could be an egg bound female, or it could be bloat. Since the fish isn't eating as well as having the distended belly, I would be concerned.

Can you isolate her and add some epsom salt at 1 cup per 100G? If she doesn't start eating again within 24 hours after doing this, you might need to treat for bloat.

I'm sorry you lost her.

I would keep a very close eye on the remaining fish for signs of problems.

She may have been egg bound, but it would depend on how long she had been that way whether she would have succumbed to it or not. If her abdomen just became distended prior to you posting here, I seriously doubt that was the problem.

I would watch for white stringy feces, reclusiveness, or any other unusual behaviour.

I might even go ahead and feed some treated antiparasitic food since they all have such good appetites, just to be on the safe side...