She should probably get it checked out, but if it is intermittent, could it be something like a lactose intolerance? I developed one fairly recently, with less spectacular symptoms (griping stomach pains) but my daughter has it too, and she has the same results as yours if she drinks milk or eats chocolate.
I just switched to lactose-free milk, and will go back to dairy after a couple of months to see if I have 'green out' of it. You can also get enzymes from health shops to take before eating dairy products, if it turns out that an intolerance is to blame.