LoveTeachingMath is right on. I would bet her teacher makes comments to the kids, or behaves in ways that betray his or her own stress, and the children pick up on that. It may even be that the teacher has hinted that his or her job depends on these tests (which it basically does, in many schools). Even hinting that kind of thing as a joke can upset kids this age, who do not always get the humor.
I would keep encouraging your child, keep any discussion of tests very low-key, and definitely talk to the teacher. Say you want the teacher to have a heads-up and wondered if your daughter might be worried that she won't advance, or that her teacher will be in trouble if she doesn't do well. Kids this age want to love their teachers and some of her stress may be a feeling that "I've let down teacher if I don't do well."
This is a case where it helps to be plugged in and talking with other parents of classmates. I'd want to know if their kids also are stressed or have indicated that the teacher has made any comments about the test affecting their ability to advance or the teacher's or school's own standing.
Is this your child's first year of having standardized tests? That would be part of the issue too, if she is not used to them.