If I had to put money on it, I'd say she's on an endurance-enhancing drug. She may not even know it because they take them as vitamins and supplements.
For what it's worth, I'd put my money on there being at least half a dozen other swimmers in these Olympics from other countries, possibly more, that are also doping or have in the past. I can name two off the top of my head--Jessica Hardy and Cesar Cielo. Again, they may not even be aware of it, as those substances are sometimes concealed in supplements.