If it is in the tank with other males, it may be pregnant. I would look up how to tell the genders apart. That's how I found out that I bought both males and females. I don't have platties, but I have mollies which look similar, and that's how my molly female looks when she is pregnant. Molly females have rounder pectoral fin, and the male pectoral fin is more pointed and can lie flat against the body, because it is actually their genitalia they use to impregnate the female. Male mollies, though, have larger dorsal fins. I don't know if this will help very much, but I hope it does!
Definitely a pregnant female, it looks if she could drop soon but its hard to tell, some fish or sometimes a fish will be fatter or skinnier when they give birth, they can also hold onto them for longer than you'd expect they would making a lil harder to predict.