She needs a vet visit a full work up including extensive bloodwork. Have them check liver and kidney functions, thyroid, her liver enzymes could be elevated, she could be retaining fluid, even though you say she is using the litter pan. Does she pee frequently in small amounts? Is she straining at all? Blood in her urine? I would also get some pedialyte down her if you can. You can use a feeding syringe, but don't force it quickly down her throat. If you can't find any at the store or drug store you can make your own
1 cup boiled water- cooled
pinch of salt
pinch of baking soda
1 tsp sugar.
Boil the water, remove from heat. Add the ingredients and stir well. Cool before giving to your cat.