HI Swordtail, we need some more info. What are your water parameters - ammonia, nitrite, nitrate, pH and temperature. What and how often are you feeding your fish? Could be from a lack of nutrition/vitamins.
I agree Mike. I read an article that said live bearers need lots of vitamin c to prevent this same thing. Now I feed mine veggie flakes once a day and have had no more of that type of problems. But it won't help after it has already happened.