Could be bloat. Could be pregnant but not yet showing the darkening gravid spot yet. Has she been like this since you got her, or has she gotten bigger over time? Platys can store sperm for months, so even if you don't have any male platys, she could have gotten pregnant from stored sperm. However, they only stay pregnant for 30 days or so, so if her size/shape hasn't changed in more than 30 days, she's probably not pregnant.
I'd try treatment for bloat, and see if it helps. Platys need veggies in their diet, and if they don't get them they could get bloated. Try feeding her peas for a day or so - smash them a bit so that the skins pop, or remove the skins. I've done both, and either way the fish eat them. I put them in a mesh bag, squeeze a little so that peas are pushing through the mesh a bit, fasten the top of the mesh bag with a plastic clip, and attach the clip to wall of the tank with a suction cup. All my fish go for them - platys, mollies, guppies, cory cats, otos. Even the snails.
Good luck with her.