I taught 2nd grade and dealt with all kinds of behaviors. Whenever I noticed a shift like this (great student, to one who misbehaves/talks back), I normally found it was because of one of two things... either something happened at school that had a big impact on the child, or there was something going on at home, that was affecting the child more than anyone knew.
I am not sure where you are from, but in March 2nd graders took standardized tests, which could have stressed your daughter out. Also, toward the end of the year, things get more difficult to prepare children for 3rd grade.
Perhaps you can discuss this with her teacher and see if he/she knows what might have happened.
The second thing I would do is take a look at your family life and see if there has been any changes to that.
You may want to have a heart to heart talk with her to see if she vocalizes anything.