What Are The Recurrent Episodes Of Light Headedness,dizziness And Blurred Visions?

I have been experiencing severe lightheadedness and dizziness when I stand up (especially if I have been sitting on the floor). I experience this whether I have been eating sufficiently or not that day, drinking plenty of water, drinking alcohol or not, smoking marijuana or not, or smoking tobacco or not, or drinking coffee or not. I do not abuse any other types of drugs. I lead an active lifestyle and I am at a healthy weight for my size (150lbs at 5'9"). I have tried cutting out alcohol, caffeine, tobacco, and marijuana to no effect (I will still get dizzy). I do not take any medication. I have, in the past, taken supplements, but I do not recall any effect while taking this one way or the other. Several years ago, I would experience severe dizziness which would cause me to faint. This has not happened in at least three years. Recently I have had mild headaches that do not respond to regular doses of acetaminophen.

This issue will require me to sit down for a minute or more. My vision will get blurry or sometimes I will experience tunnel vision. I have not seen a GP about this issue, but I am starting to get concerned that this may be indicative of a more serious issue. Can you please advise?