Is her weight loss destructing her body?

My sister has lost a ton of weight in a very short time. It's been two weeks and she's dropped nearly thirty pounds. She runs and swims every day, doesn't eat much, and is usually so tired she can't sleep. She is always complaining about sore bones too. Lately she has been having trouble controlling things like her bladder, twitching, and loss of vision. She hasn't eaten for three days because it makes her "feel sick." Is there something going on?