In this book, Karana hurts her leg in the time after she has already been left on the island all alone. You can find this right at the end of Chapter 13.
She ends up hurting her leg when she has decided that she is going to try to kill one of the sea elephants because she wants his tusks. While she is near to the bull sea elephants, two of them start to fight.
One of them starts coming towards her. She backs away, trips over a rock and falls. She hurts her leg quite badly when she does this.