I often refer to such children as 'anxious worriers'. Now, some children come into the world with this temperament. They are not displaying emotional disturbance, but rather a basline temperament given to nervousness. You can read about such children in Stanley Greenspan's book THE CHALLENGING CHILD. On the other hand, it could be that your daughter is displaying an anxiety disorder (likely Generalized Anxiety Disorder). It wouldn't hurt to have her evaluated, and at the very least you and the clinician can collaborate re: tactics to help your daughter manage her anxiety.