Can you see any swelling anywhere on her leg, any redness? From what you say, it sounds as though she has injured her leg or her foot.
Start by the easiest to observe, the foot. See if the pad is swollen or red, if there is an infected callous on the bottom of her pad, or any cuts, and whether her toes are all OK -- not bent, or limp, or discoloured.
Then check the leg. Gently feel the length of the bones between forfinger and thumb, and look for any differences from the good leg. If there is a fracture (unless it's very high up) it is often possible to feel the point where the fractured bones overlap slightly. If you don't feel a fracture check for a warmer area, which would indicate an inflammation.
Let us know what you find out.