Isolate her so you can monitor and try to figure out what is going on, and to prevent injury by other birds. If you can get a fecal float test done by a vet that would rule in or out internal parasites and would be the best option for that rather than guessing. Internal parasites are a common cause of weight loss in an otherwise healthy bird. Monitor her eating and drinking, is it normal? Are droppings normal? Is her crop working properly, should be full in the evening before roosting, empty in the morning. If it's very hard, doughy, or large and soft/squishy she may have a crop or digestive issue. How old is the bird and was she laying prior to getting ill? Feel her abdomen below her vent, between the legs and see if if feels normal or if it feels swollen and maybe water balloon like. You can compare with other birds if not sure. Also check her over well for any signs of lice or mites.