We'll, either she is pregnant. If that is the case, maybe get a breeder box 7$ at petco. Others would disagree with me on that, but it works for me.
She could be constipated. If that is the case, try feeding smashed, cooked, and de shelled pea that is not flavored (I buy frozen) and make sure it is really small pieces fed.
She could have dropsy.
You can do an EPSOM salt bath - not aquarium salt - because epsom salt eases the tension of bloating while aquarium salt actually retains water. Epsom can be used for both constipation and dropsy. The prognosis for constipation is better. Dropsy accompanied with pineconing, not so much, but it can't hurt to try. Google the exact procedure. I don't want to give you wrong info.
I've had success with the bath, but I've figured out the best ratio of salt:water:time for my particular fish.
Make sure the water is pristine.
If you are concerned she is isolating, perhaps you could put her in a QT or makeshift QT so as not to exasperate her with fish.
If she is going to give birth, you may see some behavioral changes. My platy puts her body in a weird position, isolates, darts around in circles, and a variety of other things that she doesn't normally do. That is when I know to put her in the breeding box.
Obviously, each fish is different.
