Literally what this forum is, most people on here will have exactly the same issue but no one knows 100% why.
It does seem that some people have a visual issue where both of their eyes dont function well together, a binocular vision issue (i believe it is called)
Other people seem to have a neurological issue and / or migraine issue and are more affected by the various flickers modern technology exhibits, be it PWM or some sort of color space flickering to mimic a higher color space.
Some people may have both.
My advice, try a poor color reproduction screen, try an older screen. Something flicker free but TN as they are usually not very bright. BUT if it is the graphics card / nvidia / intel / AMD then it could be a driver issue, or just how they are now.
I have many machines I cannot use due to similar issues.
It may be the card operates in a way you cannot tolerate, you may be able to get used to it or you may not. Make sure its not making the screen much brighter than before, I had that on a laptop where the external screen was a different brightness on that laptop but everything else it was dimmer - not sure why.
Try limited color output in Nvidia and maybe lower the brightess in the nvidia control panel as well. Also, you can try on your monitor set the brightness / contrast to what you normally have, but then set the R G B color values from whatever they are now (say 50 each) down to 25, this will dull the color brightess somewhat. - worth a try.
ALSO, be very very sure the card is fitted 100% correctly in the PCI-E slot, fully seated in there, not slightly out etc, I also assume your PSU is adequate for the cards requirements. Both of these things could have a negative impact on how its working. (trivial but worth mentioning)