You have symptoms of orthostatic hypotension (also called orthostatic intolerance). Orthostatic hypotension is a form of low blood pressure; the blood vessels do not constrict (tighten) as you stand up. This condition is usually a symptom of an underlying condition rather than a disease in itself.
The drop in blood pressure reduces the flow of oxygen to the brain. Symptoms include dizziness, lightheadedness, altered vision (blurred vision, white outs, gray outs, black outs, double vision), fainting, anxiety, exercise intolerance, fatigue, headache, heart palpitations, sensation of difficulty breathing or swallowing,
sweating, weakness, nausea, mental confusion, muscle tremors, hearing disturbances (including impaired hearing, crackles and tinnitus), pain in the neck/shoulder, low back pain.
There are numerous reasons for orthostatic hypotension such as dehydration (eg: due to vomiting, diarrhea), fever, anaemia, diabetes, adrenal insufficiency, prolonged bed rest, hypothyroidism, autonomic dysfunction, significant blood loss, heart problems, medications, anxiety and panic attacks, acute hyperglycemia, hypoaldosteronism, hypokalemia.
Make sure you're hydrated and are eating enough food. If you are, these are symptoms of autonomic nervous system dysfunction, which is common in young people but poorly understood by most doctors.
Hello~I agree with the other two posters, it sounds like you have orthostatic hypotension (also called orthostatic intolerance). Red_Star has explained perfectly.
My suggestion is to see your GP to have any serious issues ruled out.