keep talking to her and try other ways in which you feel she is comfortable with her expressions. you could play with her and do role play for preschool teacher and kid. or you could help her sit with drawing and see if she can draw something that gives you an indication.
whatever you do, try and get it out of her through actions and words. if nothing helps, make sure you do immediately also speak to the teacher and staff. most of them also have CCTV these days, so maybe you can look at some footage around those days when she got upset.
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