Hi, If I Sit Down For A While And I

Hi, If I sit down for a while and I stand up, my head feels like it just pressurised and I can feel my heart beating inside my head, which causes my hearing to be reduced. Everything goes back to normal after a minute or two but I have also started getting a whistling sound in my ears pretty well constantly, this started about the same time the pressure in my head started. Can you please advise on this, kind regards, Andy