I think my daughter is stressing too much

I agree with your observation about your daughter feeling stressed.  

Are you able to open this topic in conversation with her?

Also, reflect on your own expectations as a parent.  

It is possible that your daughter is trying to please you by getting consistently high grades.

If your daughter prefers talking in confidence to a therapist, then this may help her regain a sense of balance in her life so that schoolwork feels less stressful.

I wouldn't take her to a doctor because based on what you write, the problem is psychological and emotionally based.  While the stress may have physical symptoms, addressing the root cause of the problem has nothing to do directly with something being physically wrong with your daughter.

Unless there is some other medical or physical problem that would explain your daughter's sense of stress,  I'd start first by bringing your areas of concern to your daughter, then possibly to a therapist.