Hi there, it sounds like you might possibly be affected with orthostatic hypotension, since you have been getting lightheaded associated with position changes. It can happen often in young people with low blood pressure at baseline, especially if you are a little tired or dehydrated. I would recommend you make an appointment with your primary care doctor about this problem, as sometimes he or she can prescribe you a medication (like midodrine) or a stimulant to help you with this. They can also run tests on your heart (in case the hospital didn t when you went) to make sure there is no cardiac cause. In the meantime, I would recommend that you make sure you stay well hydrated, and of course please do not continue walking or staying upright when you are lightheaded and dizzy in order to avoid fainting again. I wish you all the best and hope my answer was helpful for you.