I am going to start this saying that not knowing your daughter I really have no idea, but here are a few things to think about. I would wonder if it's stress, anxiety, and/or confusion about what is going on. Does you daughter know why she is being tested? Perhaps she is confused and not getting the correct story. This seems to be the plot of numerous children's books where the protagonist misinterprets something the adults are doing. Your daughter might feel like she has lost control or say to what is going on or when it's happening. I myself remember a time in 3rd grade where I was horribly upset that I was being pulled out to do "different" math with a bunch of horrid boys and separated from my new best friend. (Different math turned out to be the top math group. ;-) ) Perhaps your daughter is missing something she loves when she was taken out for tests. Maybe other kids are teasing her about it.
