How old is she? Is she old enough to verbalise? Can she tell you why she is upset?

Does she have to go? How long are the days? Can you go and stay with her a couple of days? It might give you some insight into what it could be that's upsetting her. I know some say that you shouldn't do this but this helped my son recently when he started disliking daycare (incidently, after we dropped from 2 days per week to 1 day per week). I would hang around for half and hour and chat with the carer, play with him, etc. Then I would say 'OK i'm going to go now, are you going to stay?' and he always chose to stay. It was a gentle option that worked for us.

If it were me & my DD, that behaviour would ring alarm bells, especially if it hasn't improved in the 5 weeks and after coping fine with separation in day care.

Trust your gut. You know her best.