yeah kuan, acclimatization kind of thing.. i only get it when super cold. the scarf thing helps but being/warming up in the cold air before hitting it is definately a must.. just walk a bit.. find a hill.. those are the best.
the air maybe warm when it finally gets to the lungs but restricions happen in the brioncal tubes and the air can still be cold there.. just my own experience..
learn to leyer and how to work them.. avoid heavy sweat.