Hi. I Have Been Occasionally Having An Issue When Standing

Hi. I have been occasionally having an issue when standing up at night. After getting up, I take a few steps and begin to get severe nausea. This is accompanied by starting to sweat, I also feel like I getting a little dizzy. I go from feeling normal before standing to severe nausea. It primarily happens when I get up to use the lavatory during the night. Any ideas? I am a reasonably healthy male in mid-forties