Suggest Treatment For Dizziness While Standing Up

Hi Doctor, I am 26 yrs old, athletic, i work very long hr sitting and using a computer(sore muscle on my middle back, lower back, and neck). Things started with I felt dizziness, lightheaded when i stand up from a seat. When i sit, i do not feel lightheading. My dizziness and lighthead goes away immediately after i start to walk. It has been 2 weeks, i went to see a doctor recently, doc checked sugar blood, blood pressure sitting, standing and laying, all normall. Cervical spine x ray is normal as well EKG is 45, primary care physician refer an elec cardiologist. Cardiologist check heart rate 90 after i was asked to walk the hall way. Cadiologist thought my dizziness is likely orthostatic low BP, asked me to increase fluid and salt intake. Its been 3 days since i increased my fluid and saly intake, i still felt dizzziness when i stand up, please help! Many thanks!