I suspect it's not just from bad vision, because I've had problems in visually busy places even as a child, before I needed glasses. I didn't get headaches then, but I had a habit of looking at the ground all the time, which my parents taught my not to do once I got older because it looked weird. I recently tried out looking down all the time in busy places, and found it made me feel a lot calmer, and my head feel less tight. So I suspect that looking down was a way of avoiding looking at other stuff. (I decided it was too costly to use in most cases, though, because it looks really weird.)

Thanks for the suggestion, though. I guess it is possible that blur circle also contributes.