I am not a trained medical professional, so please DO NOT take ANYTHING that I say as definitive.

7 bottles of water per school day could be an issue. Please do not think that it IS an issue until a medical professional states that it COULD be an issue.

I ask about "very flexible joints" because very rarely your symptoms COULD be attributable to you having a connective tissue disorder. Once again, DO NOT perceive my questions NOR answers as definitive about anything.

Very flexible joints can RARELY be associated with symptoms similar to yours. Another poster mentioned POTS which COULD BE what you are experiencing.

Regardless, standing up and nearly blacking out is uncommon. While I was still growing into my 6'4" frame, I experienced similar symptoms. Post puberty, it never occurred again. I am still undergoing a workup to achieve a diagnosis for my issues. My early doctors thought that my low blood pressure as an adolescent were nothing but good news. However, it could be a sign that your blood pressure is too low such as what occurs with POTS. Again, DO NOT read into anything that I have said as being definitive. There are many explanations for certain conditions and none of them should worry you until a medical professional tells you that you have something to worry about. If your symptoms disrupt your life, then please seek professional medical help.

I asked my questions for further clarification for all who read this thread. Please do not read into them that anything in particular is wrong with your body. Hopefully soon, some actual medical professionals will chime in with their opinions. Until then, do not worry about anything other than taking your time when standing up so you don't experience your symptoms of getting up too quickly.