Hi S.,
so sorry your little one is going through this.
My 3 year old just started pre-school. We started a ritual each evening where we go over our day and talk about what made us happy and what upset us...I have learned a great deal about him this way and things i could not know. Like that he was upset when a little boy in the playground would not share a toy...etc...
Maybe if you talk to her and enable her to share with you this stuff, you can figure out what happened and help her deal with it.
Maybe she fell and the teachers did not tell you?
Maybe she feels left out from the other children and she cannot express it?
Maybe one child acts up and it scares her????
Try as well to hang out for 2 minutes in the morn at the class to see if you can sense anything. I would also ask the teacher.
I hope this helps.
Jilly