Hi Dear user !I have gone through your question and I understand your concern.
According to what you say, it could be caused by low blood pressure .
The symptoms shows when you are standing up,right?
Your condition may come from the heart, endocrime system (
low blood sugar),
dehydration, nervos system.ect so , I recommend to go to your local doctor as soon as you can , he will check your heart,
blood test and other examinations to find out themain cause ..
Meanwhile I suggest to measure your blood pressure while you sit down and after that when you stand up , if there is any changes between then , more than 20mmhg in systolic blood pressure or more than 10 mmhg in diastolic blood pressure you may have orthistatic blood pressure .
Stay hydrated and avoid standing up for a long time or any other movements that make you feel the symptoms.
Wish you good health.
Thank you for using HCM.
If you have any other questions please feel free to ask me.