I know of a few pilots who suffer "Breakaway" which is a vertigo or un- easy feeling whilst above say 1000-2000ft agl. Interestingly enough these pilots have jobs which keep them very low to the ground for 99% of their jobs.
Some people have surmised its lack of visual stimulus that brings on these feelings.
I have been a bit queasy while being PIC and had to cut a flight short, but that was in my earlier days with head firmly inside the cockpit... it hasn't happened for a while, nothing to do with alt!