Hello, I am an 18-year-old male. My BMI is normal. Whenever I stand up from laying down, sitting, crouching, or kneeling I experience extreme coldness in the back and left side of my skull. I also become disoriented and confused, and cannot speak for up to two minutes. Also, my vision goes completely black for up to a minute and I lose muscle control. Oftentimes I will stand up and have to stay put for a minute until I can move again or I almost fall over. Does anyone know what could be wrong?