Horses, like children, also have their good and bad days - and many young horses go through that terrible two stage or aren't consistent in their behaviour/attitude.
It sounds like she could be getting bored, or perhaps feels like there's too much pressure on her - perhaps she is being trained too extensively or too regularly? At that age, they have very short attention spans, and you constantly need to provide them with new 'puzzles' to work out to keep their minds stimulated. Lunging is something that is, to most horses, irritating and dull, so this could be it. I would give her a break from it and try something else and groundwork exercises, such as Parelli, are great to keep young horses listening and focused.