Any chance you've recently fed her peas? My baby platys have "choked" on them when I haven't smashed them small enough. My other fish have done it when they stuff their mouths with algae wafers that have also not been broken up enough.
If you are going to do a salt bath, make sure it is the right kind. Unfortunately, I don't know off hand if it is aquarium or epsom salt, but they have opposite results.