My first thought is that if your DD is being challenged, it may be the first time she has experienced frustration.

My DS7 becomes frustrated or tunes out when he hasn't solved a problem instantly and correctly the first time.

Also, you can't underestimate physical and developmental changes. Over the years, I have discerned patterns where DS regresses significantly in coping skills- once it calms down, he's either grown two inches, or acquired a new skill set.