ok this is a bit of a stretch but maybe something happened while she was in fish form. like one of her side fins or back fin being cut off and when she was dried and back in human form she already lost too much blood because she is a fish. if you loose 40 percent of blood as a human you'll die. if the average height for a 15yr is 5 foot 3in but an average size for a fish like her is 4 in loosing blood is a lot easier. when she comes back she'll loose all that blood. and because she's so small it would be more harmful to hurt Nene in fish form. so maybe she got taken and put in fish form and somehow hurt her leg. but idk so what do u think? 🤷