ORIGINAL: downunder
It's the background (clouds, trees) wizzing past that tends to make you dizzy because your eyes flick all over the place trying to lock on to something.
Here in Chester, U.K. we have 2 or 3 sites to choose from on a Sunday.
One of them is a small sports field with trees on 3 of the 4 sides.
Although we are mostly pretty experienced flyers, this site still has a much higher "dizzy" factor compared to the larger, open view fields.
Funnily enough, at this site, we experience "disorientation" even when not "going round and round", like when flying pattern or combat.
Generally, focus on the model, and after some hours practice it stops happening.
I have noticed that after a few years "time off", I thought it wouldn't - re-appear, but it does, it's not "like riding a bike" !!