A little more info would help.
What are your water parameters (Ammonia, nitrite, nitrate, pH)?
Are the fish eating/swimming normally?
Are there male swordtails in there too?
Are the scales sticking out like a pinecone? Are the eyes bulging?
Without this info and/or pics, it's hard to say. However, possible causes/steps to take are:
1 They're overfed or constipated/feed them a partially boiled, shelled pea and cut down on the amount of food you feed them.
2 They're expecting babies/congratulations!
3 They're suffering from an internal bacterial infection/quarrantine them ASAP
and treat with kanacyn