If you normally wear flats, wearing high heels can be horribly painful after a while because most or all of the weight that would be spread across the heels is shifted to the balls of your feet, plus you're in a different posture and thus having muscles stretched and compressed in odd ways. Even with shoes that fit perfectly, it has much the same effect as changing to a job that requires standing for hours at a time -- you get used to it after a while, but initially that extra pressure on your feet leaves them very sore.