As Olive says your GP will be able to arrange allergy testing. As for her immediate diet, I would take her off dairy first and see if that makes a difference, and if it doesn't then try a gluten free diet. I was also advised by many other parents, and my chiro, to take my youngest off both gluten and dairy but I just took him off dairy to start with and his health has improved so much that we have not had to consider a gluten free diet. I admit I did it this way because a dairy free diet is easy, but gluten free requires a lot more work and money and I didn't want to do it unnecessarily. I also think it is quite hard to isolate if there is only one intolerance or allergy when you remove two foods groups at the same time.

If you do put her on a gluten free diet then do it in conjunction with a GP or peadiatrician who can monitor her and ensure that it is gluten intolerance, and not coeliacs disease.