Are the scale pineconing? Nice pic from up would be nice. It may be constipated, just bloated or with eggs. How much & what you feeding them? Give them some daphnia.
To me it just looks like an egg filled female...unless, like Sprinkle said, you see "pineconing" of the scales. Try alternating foods & feeding a bit less or a bit less often. Some fish tend to overeat given the oppotunity...