Thank you everyone!
Crimson, that platy is one of two that I just acquired recently. She is definitely smaller/thinner than the rest of them, though I don't know precisely what you mean by sunken belly. My others (like the red wag) have all fattened up quite a bit since I got them, but she hasn't been here nearly as long. I'm fairly new to the fish world. She is eating well and acting normally. What should I do and/or be concerned about?