my guess would be that you have some type of digestion issue. Without tests being run, it would be hard to really know whats actually wrong.
It could be a food allergy as well.
If your mother wont take you to a doctor there is not much you can do. You could speak to the nurse or guidance councilor at school, but they will just call your mom.
Without knowing anything. what id suggest is to not eat or drink anything with milk in it. its possible that you are lactose intolerant. So dont have any dairy and see if that helps. And tell your mom you NEED to see a dr.
What are you eating? And do you take a shower every morning? Just simple.questuons.to.get to the heart of the problem.