Not sure what that could be. Could be something simple like muscle tension that just suddenly relaxes. It's not likely that it would be a blocked artery that released after smoking. If diagnostsic tests have shown nothing, that's a good sign. But if it keeps up--or if the chest pain keeps up and worries you--go on back to the doc and ask about it. Sometimes with neck-related matters like occluded carotid arteries, the best diagnostic tests are simply listening to the blood flow with a stethoscope and then a sonogram. If you have any family history of arteriosclerosis (blocked arteries) or high cholesterol, it might be worth checking again.