Experiencing Dizziness and Lightheadedness - Need Help!

I have a few symptoms that im rlly confused by. 1. When i stand for more than five minutes, sometimes a few seconds, i start getting dizzy, lightheaded and sweaty and i can feel my heart sometimes 2. I can't stand straight otherwise i'll fall over/trip so i stand with one leg bent and this somehow makes me not feel dizzy 3. When i stand up, quickly or not, everything goes black and i feel super lightheaded My heart rate was like 130-151 for a while after i stood up These are all worsened by heat Another few are: I occasionally feel as though everything is spinning and moving and tilting sideways I sometimes feel like certain limbs are growing and shrinking even though they aren't I have these moments where i zone out involuntarily in which my eyes ALWAYS cross without me moving my eyes Why does any of this happen?