You mention she is eating a healthier vegetarian diet than most teenagers - does this mean she is eating live/raw foods or just avoiding meat? The reason I ask this is because it sounds as though she may be lacking the enzymes she needs to break her food down, which means it is fermenting in her stomach. This would cause pain and bloating. It could also be an ulcer or a bad yeast infection. If it is enzymes, then adding papaya or some other enzyme to her meals will help her break down her food. Also, making sure half of her diet is raw fruits and vegetables will help to get live enzymes into her diet.
If you give me more information on her diet and how often her bowels move each day, it will help us narrow it down.