Normal brain response i guess.
I once had to hover an AS350 first at 1,000feet then we moved up to 3,000feet, started to feel a bit uneasy, up to 6,000feet we went,now it didnt feel good at all. Finished at 10,000feet hovering for almost one hour. I was really uncomfortable, and its difficult to hold the aircraft still in a hover at that altitude, looong way to the nearest reference point. We were monitered by a radar and as soon as I started to move they would call up and ask us to hold still. After an hour of this I had a wet grip of the controlls and was really tired(squeeze). We decended down to 3,000feet again for another hover and wouldnt you belive it,it felt comfortable all of a sudden at that altitude. Normaly I fly att 1,000 feet and once you get up there its a whole new world.