This most sounds like an injury. You would not believe what cats get into when you are not around. Have her checked out by a vet. Then you will know for sure. Let's hope the poor animal is not suffering too much
Why Is My Cat's Right Side Hip Swollen?
Maybe your cat has been fighting? Or something has happened to your cat. If its really bad, go see a vet. My cats fight a lot. We have taken them to vets and have only lost one to a broken leg.