It sounds to me that your daughter has told you what's causing her sickness. She's being bullied. This has become a common place problem at school, and I won't go in to my opinion on why. However, I would suggest that you contact her teacher and make an appointment to discuss this issue before it gets out of hand. The fact that you've told her she's special and that you're proud of her is wonderful, but it doesn't solve her problem at school.
This type of situation needs to be dealt with quickly and appropriately by the authorities in charge. On the slight chance that talking with the teacher doesn't help, be ready to contact the principal if needed. Many schools today have programs in place for dealing with bully situations. If your school doesn't have one in place, it should adopt one as soon as you talk with those in charge and they find out they have a problem.