It might also be worth having her checked out for a urinary tract infection. When my son was at pre-school nursery the staff told me that he was sitting at the side, crying and seemed generally down. He was also pretty lethargic and easily distressed, but quietly so, at home.
To rule out any physical problems first I took a urine sample to the doc, and it turned out he had a urine infection. I was surprised as he hadn't been in any pain, and seemed to have been peeing normally. The doctor told me that in young children, and elderly people, UTI's can often be overlooked and mistaken for depression.
It might not be the cause in your daughter's case, but well worth getting it checked out, especially as very easily and quickly treated with antibiotics.