What iPhone do you have?
If it’s an iPhone X, XS, 11 Pro, 12, 12 Mini or 12 Pro, you could be suffering from the PWM (pulse-width modulation) system their OLED displays use to regulate brightness.
Basically the OLED pixels on iPhones flicker on and off very quickly, and the lower the brightness the faster the pixels flicker. This is how OLED iPhones regulate their screens’ brightness.
Now, this flickering is far too fast to see with the human eye, however some people (approx 1/10) can actually feel nauseous after extended use, you could be one of them.
From what I’ve read, Apple lessened this affect on the iPhone 11 Pro’s last year, however I don’t have any data on the newer models.
If this is in fact the issue, you may want to consider replacing the phone with an LCD model like the SE, XR or 11 as their screens are perfectly still.