My guess without being able to feel it or see her walk is the bursa which is the sack that holds the fluid to cushion the joint is enlarged either due to injury or infection. If its an enlarged bursa this could be caused by a strain, chip or a fracture. Fever? Is walking normally? Does it feel soft or hard? any breaks In the skin? If she was mine I would load her in a trainer and take her to the closest vet hospital to get it x-rayed.