Q: physically, how are you watching them? On a couch? Lying in bed? Hunched over on the bus? Phone? TV? Do you have auto motion plus enabled (you monster)? Do you watch it in one place? Has that room been checked for carbon monoxide? Is it a basement? Has it been checked for radon?
It could be a lot of things and there’s not enough information provided but the first thing that occurred to me is blood pressure. Do you have a blood pressure cuff? Try to use it when you’re feeling light headed and when you’re not and comparing numbers.
It could be a physical things about the position of your body, pressure on something because you’re hunched over or something. It could be a visual things too. There are conditions where moving/changing visuals could affect you.
It’s kind of overwhelming all of the things it could be and you really should go to a doctor.
While you’re waiting for your appointment try to make observations about what is the same about reading vs watching and what is different. This will help them figure out what to investigate.