What Causes Feeling Of Inhaling Ice Cold Air While Breathing?

from time to time i feel like when i breath i am inhaling ice cold air. any idea what that could be? i have been to er twice becouse orig thought i was havin a heart attack.but took xray both times & said everything looked fine & my lungs sounded just fine.this ice cold breathin feeling had started couple weeks after.also i am 39 years old & have gave up smokin 4 months ago but did smoke over a pack a day for 20 years.seem like these symptoms started shortly after i quit smoking.i also feel that i have to vocus on breathing at times.any sugestions would be greatly appreciated. thank you!!!