Computer monitors and brain tumors...and no, not that...

When walking around in the office looking at a computer monitor several yards away, the screen area of the monitor seems to drop slightly below where it should be as I my feet hit the floor with each step, almost as though it has inertia in my field of vision. Why is this?

The obvious answer is brain tumor. Setting aside hypochondriac answers, what’s going on? I wouldn’t think it has something to do with the frequency of the monitor, and I assume my eyes are firmly planted in my head. I’ve not noticed such a phenomenon with TVs or anything else.

Anyone else experienced this? Suppose it’s indicative of a physical problem or the physics of monitors?