The screen on a phone is heaps smaller than on a laptop or computer. It's perhaps causing the extra strain because of the size and the format.

Phone screens tend to swing. It might sound strange but I have problems with waiting at the lights and having the traffic swoosh past me from right to left and left to right. I don't even need to move my head. It's a visual thing. (But then I have vertigo a lot of the time anyway)

Do either of you have a history of migraine at all?

Edited to add: roralea, I just noticed your signature and you have Meniere's. I take it this feeling of motion sickness you posted about feels differently from that type of dizziness?