Sometimes when I stand for too long, I get nausea and stomach cramps. Sitting down or walking cures that. Dunno what triggers that, it's not a consistent effect, just sometimes.
Nope, I always forget it when I go for a check-up because it happens so infrequently. I had that since I was a kid, I remember throwing up at my First Communion because it took forever for the pastor to stop reading from the book.
Are you locking your knees? That can make people light headed and pass out. Idk. I'm sure you've thought of it but I'd hate for the solution to be that simple and you not know. I guess the question is does it happen if you're walking for a long time or just standing still?