Hi, I would send her to her room for some alone time, when she calm's down, ask her what's wrong? Do not put the thoughts into her head, in other words don't say (is someone bothering you at school) let her talk when she is ready. And go to the school and ask the teacher about anything which may have happened at school. When our daughter was in pre-school we got a paper everyday outlining her day, what she ate, how much, or how little she ate, her mood, happy, tired, cranky, playful, does the daycare do this for you? If not ask them to, talk to other mom's, and or dad's who have children at the daycare she is in. Go to local libary and check out a book on how to handle stress, in constructive ways, everyone has stress, she just does not know a good way to work through the feelings she is having. Paint, draw, breath, I use to lay in my yard and look up at the clouds going by, and running, I loved to run, exercise is a great stress reducer. Walk the dog, throw rocks into the pond, or pennie's into the water at the park. Soon you'll find what helps to calm her down. Best of luck to you, & your niece.
Check out anything that may be happening at the play group children copy behavior they see, can you drop in unannounced , does the play group play with the children are they stimulated enough.I fshe has started this recently it is possible it comes form there. Good Luck